avcodec/pnm_parser: Avoid reparsing failed data.

Alternatively the parser could be re implemented / redesigned so as to better
and more efficiently find frame boundaries

Fixes: Timeout
Fixes: 9210/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PBM_fuzzer-4770771833454592
Fixes: 9214/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PPM_fuzzer-5741633353023488
Fixes: 9219/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGM_fuzzer-6249230237696000
Fixes: 9550/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PAM_fuzzer-5312669836902400

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

libavcodec/pnm_parser.c

@@ -51,8 +51,9 @@ retry:
             if (pc->index) {
                 pc->index = 0;
             } else {
-                buf++;
-                buf_size--;
+                unsigned step = FFMAX(1, pnmctx.bytestream - pnmctx.bytestream_start);
+                buf      += step;
+                buf_size -= step;
             }
             goto retry;
         }