avcodec/pnm_parser: Check image size addition for overflow

Fixes: assertion failure
Fixes: out of array access
Fixes: 32664/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-6533642202513408.fuzz
Fixes: 32669/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-6001928875147264

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 79ac8d5546)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pnm_parser.c

@@ -111,7 +111,7 @@ retry:
     } else {
         int ret = av_image_get_buffer_size(avctx->pix_fmt, avctx->width, avctx->height, 1);
         next = pnmctx.bytestream - pnmctx.bytestream_start + skip;
-        if (ret >= 0)
+        if (ret >= 0 && next + (uint64_t)ret <= INT_MAX)
             next += ret;
     }
     if (next != END_NOT_FOUND && pnmctx.bytestream_start != buf + skip)