Fix bad return val when skipping past end of image

Fixes #439

ChangeLog.md

@@ -17,6 +17,8 @@ similar fix, but it did not cover all cases.
      - `jpeg_skip_scanlines()` now throws an error if two-pass color
 quantization is enabled.  Two-pass color quantization never worked properly
 with `jpeg_skip_scanlines()`, and the issues could not readily be fixed.
+     - Fixed an issue whereby `jpeg_skip_scanlines()` always returned 0 when
+skipping past the end of an image.
 
 
 2.0.5

djpeg.c

@@ -5,7 +5,7 @@
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * Modified 2013 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2010-2011, 2013-2017, D. R. Commander.
+ * Copyright (C) 2010-2011, 2013-2017, 2020, D. R. Commander.
  * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
@@ -708,7 +708,12 @@ main(int argc, char **argv)
                                           dest_mgr->buffer_height);
       (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
     }
-    jpeg_skip_scanlines(&cinfo, skip_end - skip_start + 1);
+    if ((tmp = jpeg_skip_scanlines(&cinfo, skip_end - skip_start + 1)) !=
+        skip_end - skip_start + 1) {
+      fprintf(stderr, "%s: jpeg_skip_scanlines() returned %d rather than %d\n",
+              progname, tmp, skip_end - skip_start + 1);
+      exit(EXIT_FAILURE);
+    }
     while (cinfo.output_scanline < cinfo.output_height) {
       num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
                                           dest_mgr->buffer_height);
@@ -744,13 +749,24 @@ main(int argc, char **argv)
     cinfo.output_height = tmp;
 
     /* Process data */
-    jpeg_skip_scanlines(&cinfo, crop_y);
+    if ((tmp = jpeg_skip_scanlines(&cinfo, crop_y)) != crop_y) {
+      fprintf(stderr, "%s: jpeg_skip_scanlines() returned %d rather than %d\n",
+              progname, tmp, crop_y);
+      exit(EXIT_FAILURE);
+    }
     while (cinfo.output_scanline < crop_y + crop_height) {
       num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
                                           dest_mgr->buffer_height);
       (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
     }
-    jpeg_skip_scanlines(&cinfo, cinfo.output_height - crop_y - crop_height);
+    if ((tmp =
+         jpeg_skip_scanlines(&cinfo,
+                             cinfo.output_height - crop_y - crop_height)) !=
+        cinfo.output_height - crop_y - crop_height) {
+      fprintf(stderr, "%s: jpeg_skip_scanlines() returned %d rather than %d\n",
+              progname, tmp, cinfo.output_height - crop_y - crop_height);
+      exit(EXIT_FAILURE);
+    }
 
   /* Normal full-image decompress */
   } else {

jdapistd.c

@@ -413,10 +413,11 @@ jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines)
 
   /* Do not skip past the bottom of the image. */
   if (cinfo->output_scanline + num_lines >= cinfo->output_height) {
+    num_lines = cinfo->output_height - cinfo->output_scanline;
     cinfo->output_scanline = cinfo->output_height;
     (*cinfo->inputctl->finish_input_pass) (cinfo);
     cinfo->inputctl->eoi_reached = TRUE;
-    return cinfo->output_height - cinfo->output_scanline;
+    return num_lines;
   }
 
   if (num_lines == 0)