Never free NULL in the Custom Edition loader

The game's free (debug_free) reads a header before the pointer, so NULL
faults at 0xffffffe0. A Custom Edition sound that failed to decode freed
its frames with none made, which crashed Linux on loading CMT's a30 with
Custom Edition's later sounds.map; the decoded sounds, model data and
scratch buffer are now freed only when they were made.

port/linux/game/custom_edition_cache.c

@@ -291,7 +291,9 @@ static boolean custom_edition_cache_models_convert(
 			report,
 			model_data);
 	}
-	free(model_data);
+	/* (the game's free, debug_free, does not take NULL) */
+	if (model_data)
+		free(model_data);
 
 	return success;
 }

port/linux/game/custom_edition_geometry.c

@@ -813,7 +813,9 @@ boolean custom_edition_models_convert(
 	if (!globals->model_parts || !globals->model_geometry || !globals->palettes || !scratch)
 	{
 		error(_error_silent, "custom edition: out of memory for the geometry of %ld model parts", totals.part_count);
-		free(scratch);
+		/* (the game's free, debug_free, does not take NULL) */
+		if (scratch)
+			free(scratch);
 		return FALSE;
 	}
 	vertices = (struct model_vertex_compressed *)globals->model_geometry;

port/linux/game/custom_edition_sounds.c

@@ -205,10 +205,12 @@ struct frames
 	long rate;
 };
 
+/* (none may have been made: the game's free, debug_free, does not take NULL) */
 static void frames_free(
 	struct frames *frames)
 {
-	free(frames->samples);
+	if (frames->samples)
+		free(frames->samples);
 	frames->samples = NULL;
 }
 
@@ -630,7 +632,9 @@ boolean custom_edition_sounds_read(
 void custom_edition_sounds_dispose(
 	void)
 {
-	free(custom_edition_sounds_globals.decoded);
+	/* (none, when no sound needed decoding: debug_free does not take NULL) */
+	if (custom_edition_sounds_globals.decoded)
+		free(custom_edition_sounds_globals.decoded);
 	memset(&custom_edition_sounds_globals, 0, sizeof(custom_edition_sounds_globals));
 
 	return;