avcodec/pnm_parser: Use memchr() in pnm_parse()

Fixes: Timeout (45sec -> 0.5sec)
Fixes: 16942/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PPM_fuzzer-5085393073995776

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

@@ -95,8 +95,11 @@ retry:
             sync = bs;
             c = *bs++;
             if (c == '#')  {
-                while (c != '\n' && bs < end)
+                uint8_t *match = memchr(bs, '\n', end-bs);
-                    c = *bs++;
+                if (match)
+                    bs = match + 1;
+                else
+                    break;
             } else if (c == 'P') {
                 next = bs - pnmctx.bytestream_start + skip - 1;
                 pnmpc->ascii_scan = 0;