avformat/nutdec: Check get_packetheader() in mainheader

Fixes; Timeout
Fixes: 48794/clusterfuzz-testcase-minimized-ffmpeg_dem_NUT_fuzzer-6524604713140224

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b5de084a)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>