Keep track of address returned by malloc to free it correctly

When doing g2 generation on Linux, src variable was not modified in the
same way as on Windows, which caused free() on a pointer that was not
malloc()ed.

This fixes #2219.

src/cmdline_sprite.c

@@ -264,6 +264,7 @@ bool sprite_file_import(const char *path, rct_g1_element *outElement, uint8 **ou
 
 	// A larger range is needed for proper dithering
 	sint16 *src = malloc(height * width * 4 * 2);
+	sint16 *src_orig = src;
 	for (uint32 x = 0; x < height * width * 4; x++){
 		src[x] = (sint16) pixels[x];
 	}
@@ -375,7 +376,7 @@ bool sprite_file_import(const char *path, rct_g1_element *outElement, uint8 **ou
 		}
 	}
 	free(pixels);
-	free(src);
+	free(src_orig);
 
 	int bufferLength = (int)(dst - buffer);
 	buffer = realloc(buffer, bufferLength);