avcodec/pnm: skip reading trailing bytes in get_pnm()

avcodec/pnm: skip reading trailing bytes in get_pnm()

None of the keys we support is that long and other keys
lead to decoder failure. None of the values is expected
to be longer, they are all numbers or short keywords.

This simplifies the code

Fixes: Timeout (9sec->43ms)
Fixes: 15177/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PAM_fuzzer-5080556716425216
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>