fix some waypoint bugs

1. The `waypoint_stuff_name` function used the wrong length when building the waypoint name, causing the number to be written past the end of the string in most cases.  Not sure how this escaped testing.
2. Change two Assertions, which had no effect due to the comma operator, to Errors, which match their counterparts in their respective functions.
3. Make `internal_error` consistent between the campaign editor, the mission editor, and qtFRED, and prune an unnecessary string copy.

Follow-up to #7104.  Fixes #7254.

Author: Goober5000

code/object/waypoint.cpp

@@ -399,7 +399,7 @@ void waypoint_stuff_name(char *dest, const char *waypoint_list_name, int waypoin
 
 	if (waypoint_num < 1)
 	{
-		Assertion(LOCATION, "A waypoint number must be at least 1!");
+		Error(LOCATION, "A waypoint number must be at least 1!");
 		*dest = 0;
 		return;
 	}
@@ -410,8 +410,9 @@ void waypoint_stuff_name(char *dest, const char *waypoint_list_name, int waypoin
 		return;
 	}
 
-	strncpy(dest, waypoint_list_name, name_max_len);
-	sprintf(dest + name_max_len, ":%d", waypoint_num);
+	auto name_len = std::min(strlen(waypoint_list_name), name_max_len);
+	strncpy(dest, waypoint_list_name, name_len);
+	sprintf(dest + name_len, ":%d", waypoint_num);
 }
 
 void waypoint_stuff_name(SCP_string &dest, const char *waypoint_list_name, int waypoint_num)
@@ -420,7 +421,7 @@ void waypoint_stuff_name(SCP_string &dest, const char *waypoint_list_name, int w
 
 	if (waypoint_num < 1)
 	{
-		Assertion(LOCATION, "A waypoint number must be at least 1!");
+		Error(LOCATION, "A waypoint number must be at least 1!");
 		dest = "";
 		return;
 	}

fred2/campaigntreewnd.cpp

@@ -480,20 +480,25 @@ int campaign_tree_wnd::error(const char *msg, ...)
 
 int campaign_tree_wnd::internal_error(const char *msg, ...)
 {
-	SCP_string buf, buf2;
+	SCP_string buf;
 	va_list args;
 
-	g_err++;
 	va_start(args, msg);
 	vsprintf(buf, msg, args);
 	va_end(args);
 
-	sprintf(buf2, "%s\n\nThis is an internal error.  Please let Hoffoss\n"
-		"know about this so he can fix it.  Click cancel to debug.", buf.c_str());
+	g_err++;
 
-	nprintf(("Error", buf.c_str()));
-	if (MessageBox(buf2.c_str(), "Internal Error", MB_OKCANCEL | MB_ICONEXCLAMATION) == IDCANCEL)
+#ifndef NDEBUG
+	nprintf(("Internal Error", buf.c_str()));
+
+	buf += "\n\nThis is an internal error.  Please notify a coder about this.  Click cancel to debug.";
+
+	if (MessageBox(buf.c_str(), "Internal Error", MB_OKCANCEL | MB_ICONEXCLAMATION) == IDCANCEL)
 		Int3();  // drop to debugger so the problem can be analyzed.
+#else
+	MessageBox(buf.c_str(), "Error", MB_OK | MB_ICONEXCLAMATION);
+#endif
 
 	return -1;
 }

fred2/fredview.cpp

@@ -3400,27 +3400,22 @@ int CFREDView::error(const char *msg, ...)
 
 int CFREDView::internal_error(const char *msg, ...)
 {
-	char buf[2048];
+	SCP_string buf;
 	va_list args;
 
 	va_start(args, msg);
-	vsnprintf(buf, sizeof(buf)-1, msg, args);
+	vsprintf(buf, msg, args);
 	va_end(args);
-	buf[sizeof(buf)-1] = '\0';
 
 	g_err = 1;
 
 #ifndef NDEBUG
-	char buf2[2048];
+	buf += "\n\nThis is an internal error.  Please notify a coder about this.  Click cancel to debug.";
 
-	sprintf(buf2, "%s\n\nThis is an internal error.  Please let Jason\n"
-		"know about this so he can fix it.  Click cancel to debug.", buf);
-
-	if (MessageBox(buf2, "Internal Error", MB_OKCANCEL | MB_ICONEXCLAMATION) == IDCANCEL)
+	if (MessageBox(buf.c_str(), "Internal Error", MB_OKCANCEL | MB_ICONEXCLAMATION) == IDCANCEL)
 		Int3();  // drop to debugger so the problem can be analyzed.
-
 #else
-	MessageBox(buf, "Error", MB_OK | MB_ICONEXCLAMATION);
+	MessageBox(buf.c_str(), "Error", MB_OK | MB_ICONEXCLAMATION);
 #endif
 
 	return -1;

qtfred/src/mission/Editor.cpp

@@ -2642,29 +2642,24 @@ int Editor::error(const char* msg, ...) {
 	return 1;
 }
 int Editor::internal_error(const char* msg, ...) {
-	char buf[2048];
+	SCP_string buf;
 	va_list args;
 
 	va_start(args, msg);
-	vsnprintf(buf, sizeof(buf) - 1, msg, args);
+	vsprintf(buf, msg, args);
 	va_end(args);
-	buf[sizeof(buf) - 1] = '\0';
 
 	g_err = 1;
 
 #ifndef NDEBUG
-	char buf2[2048];
-
-	sprintf_safe(buf2, "%s\n\nThis is an internal error.  Please let Jason\n"
-		"know about this so he can fix it.  Click cancel to debug.", buf);
+	buf += "\n\nThis is an internal error.  Please notify a coder about this.  Click cancel to debug.";
 
 	if (_lastActiveViewport->dialogProvider->showButtonDialog(DialogType::Error,
 															  "Internal Error",
-															  buf2,
+															  buf,
 															  { DialogButton::Ok, DialogButton::Cancel })
 		== DialogButton::Cancel)
 		Int3();  // drop to debugger so the problem can be analyzed.
-
 #else
 	_lastActiveViewport->dialogProvider->showButtonDialog(DialogType::Error, "Error", buf, { DialogButton::Ok });
 #endif