Prevent decoding a frame with inconsistent sequence and frame headers.
Fix #124, #125. Fix negative size param in pixel_copy due to inconsistent
sb128 state between frame header (parsed with sb128 == 0) and sequence
header and frame decoding with sb128 == 1. Fix
clusterfuzz-testcase-minimized-dav1d_fuzzer-5707479116152832. Credits to
oss-fuzz.