Fix standalone crash when client disconnects before mission loads

When a client connects to a standalone server and backs out before a
mission is loaded, multi_standalone_reset_all() calls game_level_close()
which fires OnMissionAboutToEnd/OnMissionEnd scripting hooks. If any
mod script calls mn.evaluateSEXP() from those hooks, alloc_sexp() crashes
because the SEXP system was never initialized (Locked_sexp_true/false
are still -1, Sexp_nodes is nullptr).

Guard multi_standalone_reset_all() to only call game_level_close() when
GM_IN_MISSION is set, and additionally guard game_level_close() to skip
scripting hooks when Sexp_nodes is uninitialized.

Fixes #7353

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: chief1983

code/network/multi.cpp

@@ -1671,8 +1671,13 @@ void multi_standalone_reset_all()
 	// NETLOG
 	ml_string(NOX("Standalone resetting"));
 
-	// shut all game stuff down
-	game_level_close();
+	// shut all game stuff down -- but only if a mission was actually in progress,
+	// otherwise scripting hooks in game_level_close() may fire into uninitialized
+	// subsystems (e.g. SEXP nodes never allocated because no mission was loaded)
+	if (Game_mode & GM_IN_MISSION) {
+		game_level_close();
+		Game_mode &= ~GM_IN_MISSION;
+	}
 
 	// reinitialize the gui
 	std_reset_standalone_gui();	

freespace2/freespace.cpp

@@ -910,7 +910,13 @@ static void game_flash_diminish(float frametime)
 
 void game_level_close()
 {
-	if (scripting::hooks::OnMissionAboutToEndHook->isActive())
+	// Only fire scripting hooks if the SEXP system was initialized (i.e. a mission was
+	// actually loaded).  If no mission was loaded, Sexp_nodes will be nullptr and
+	// Locked_sexp_true/False will be -1, so any script calling mn.evaluateSEXP() would
+	// crash in alloc_sexp().
+	bool sexp_initialized = (Sexp_nodes != nullptr);
+
+	if (sexp_initialized && scripting::hooks::OnMissionAboutToEndHook->isActive())
 	{
 		scripting::hooks::OnMissionAboutToEndHook->run();
 	}
@@ -999,7 +1005,7 @@ void game_level_close()
 		Error(LOCATION, "Scripting Mission End override is not fully supported yet.");
 	}
 
-	if (scripting::hooks::OnMissionEndHook->isActive())
+	if (sexp_initialized && scripting::hooks::OnMissionEndHook->isActive())
 	{
 		scripting::hooks::OnMissionEndHook->run();
 	}