JPEG: Handle errors when saving with libjpeg

If we are going to use my_error_exit(), we must use setjmp() to set up
its jump buffer before we start to compress, similar to what already
happened in the loading code path. Otherwise, errors in libjpeg will
result in a crash when we do a nonlocal goto to an uninitialized pointer.

Resolves: #429
Signed-off-by: Simon McVittie <[email protected]>

src/IMG_jpg.c

@@ -486,6 +486,7 @@ static int IMG_SaveJPG_RW_jpeglib(SDL_Surface *surface, SDL_RWops *dst, int qual
     struct my_error_mgr jerr;
     JSAMPROW row_pointer[1];
     SDL_Surface* jpeg_surface = surface;
+    Sint64 start;
 
     if (!IMG_Init(IMG_INIT_JPG)) {
         return -1;
@@ -499,10 +500,21 @@ static int IMG_SaveJPG_RW_jpeglib(SDL_Surface *surface, SDL_RWops *dst, int qual
         }
     }
 
-    /* Create a decompression structure and load the JPEG header */
+    /* Create a compression structure and load the JPEG header */
     cinfo.err = lib.jpeg_std_error(&jerr.errmgr);
     jerr.errmgr.error_exit = my_error_exit;
     jerr.errmgr.output_message = output_no_message;
+    start = SDL_RWtell(dst);
+
+    if(setjmp(jerr.escape)) {
+        /* If we get here, libjpeg found an error */
+        lib.jpeg_destroy_compress(&cinfo);
+        if (jpeg_surface != surface) {
+            SDL_FreeSurface(jpeg_surface);
+        }
+        SDL_RWseek(dst, start, RW_SEEK_SET);
+        return IMG_SetError("Error saving JPEG with libjpeg");
+    }
 
     lib.jpeg_create_compress(&cinfo);
     jpeg_SDL_RW_dest(&cinfo, dst);