avcodec/pnm: Optimize reading loop in pnm_get()

avcodec/pnm: Optimize reading loop in pnm_get()

Fixes: Timeout 13149 (5sec -> 3sec), 13166 (11sec -> 7sec), 13430 (5sec -> 3sec)
Fixes: 13149/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGM_fuzzer-5760833622114304
Fixes: 13166/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-5763216322330624
Fixes: 13430/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PPM_fuzzer-5758658334425088
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>