avcodec/pnm_parser: Check av_image_get_buffer_size() for failure

Fixes: out of array access
Fixes: 30135/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PBM_fuzzer-4997145650397184
Fixes: 30208/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-5605891665690624.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pnm_parser.c

@@ -109,8 +109,10 @@ retry:
         if (next == END_NOT_FOUND)
             pnmpc->ascii_scan = sync - pnmctx.bytestream + skip;
     } else {
-        next = pnmctx.bytestream - pnmctx.bytestream_start + skip
-               + av_image_get_buffer_size(avctx->pix_fmt, avctx->width, avctx->height, 1);
+        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)
+            next += ret;
     }
     if (next != END_NOT_FOUND && pnmctx.bytestream_start != buf + skip)
         next -= pc->index;