Correctly handle 0 length strings in conio (#69)

* Correctly handle  0 length strings in conio

* Fix issue where we should still update the current x,y even for 0-length strings.

* Fix issue where we should still update the current x,y even for 0-length strings.

* Remove now redundant check for length=0 on cputsxy invocation.

* cputsxy documentation cleanup.

Author: CTalkobt

include/mega65/conio.h

@@ -914,7 +914,8 @@ void fastcall cputs(const unsigned char* s);
  * @param y The Y coordinate where string will be printed
  * @param s An array of screen codes to print. Must have non-zero length.
  * @remarks This function works with screen codes only. To output ordinary
- * @warning Undefined behavior if `s` has zero length.
+ * ASCII/PETSCII strings, use the "pcputsxy" macro. No pointer check is
+ * performed.  If s is null or invalid, behavior is undefined.
  */
 void cputsxy(unsigned char x, unsigned char y, const unsigned char* s);
 
@@ -946,7 +947,6 @@ void cputcxy(unsigned char x, unsigned char y, unsigned char c);
  * @param y The Y coordinate where character will be printed
  * @param count The number of characters to output. Must be larger than zero.
  * @param c The screen code of the characters to print
- * @warning Undefined behavior if `count` is zero.
  */
 void cputncxy(
     unsigned char x, unsigned char y, unsigned char count, unsigned char c);

src/conio.c

@@ -571,9 +571,11 @@ void cputs(const unsigned char* s)
 void cputsxy(unsigned char x, unsigned char y, const unsigned char* s)
 {
     const unsigned char len = (unsigned char)strlen((const char*)s);
-    const unsigned int offset = (y * (unsigned int)g_curScreenW) + x;
-    lcopy((unsigned long)s, SCREEN_RAM_BASE + offset, len);
-    lfill(COLOR_RAM_BASE + offset, g_curTextColor, len);
+    if (len > 0) {
+        const unsigned int offset = (y * (unsigned int)g_curScreenW) + x;
+        lcopy((unsigned long)s, SCREEN_RAM_BASE + offset, len);
+        lfill(COLOR_RAM_BASE + offset, g_curTextColor, len);
+    }
     g_curY = y + ((x + len) / g_curScreenW);
     g_curX = (x + len) % g_curScreenW;
 }
@@ -590,9 +592,11 @@ void cputcxy(unsigned char x, unsigned char y, unsigned char c)
 void cputncxy(
     unsigned char x, unsigned char y, unsigned char count, unsigned char c)
 {
-    const unsigned int offset = (y * (unsigned int)g_curScreenW) + x;
-    lfill(SCREEN_RAM_BASE + offset, c, count);
-    lfill(COLOR_RAM_BASE + offset, g_curTextColor, count);
+    if (count > 0) {
+        const unsigned int offset = (y * (unsigned int)g_curScreenW) + x;
+        lfill(SCREEN_RAM_BASE + offset, c, count);
+        lfill(COLOR_RAM_BASE + offset, g_curTextColor, count);
+    }
     g_curY = y + ((x + count) / g_curScreenW);
     g_curX = (x + count) % g_curScreenW;
 }
@@ -714,8 +718,7 @@ unsigned char cinput(
     }
 
     while (1) {
-        if (strlen(buffer) != 0)  
-            cputsxy(sx, sy, buffer);
+        cputsxy(sx, sy, buffer);
         blink(1);
         cputc(224);
         blink(0);