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Bug 697703: Prevent integer overflow vulnerability.

Add extra check for the offset being greater than the size
of the image and hence reading off the end of the buffer.

Thank you to Dai Ge for finding this issue and suggesting a patch.

--- a/jbig2_symbol_dict.c

+++ b/jbig2_symbol_dict.c

@@ -629,7 +629,7 @@

                 byte *dst = image->data;

                 /* SumatraPDF: prevent read access violation */

-                if (size - jbig2_huffman_offset(hs) < image->height * stride) {

+                if ((size - jbig2_huffman_offset(hs) < image->height * stride) || (size < jbig2_huffman_offset(hs))) {

                     jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number, "not enough data for decoding (%d/%d)", image->height * stride,

                                 size - jbig2_huffman_offset(hs));

                     jbig2_image_release(ctx, image);