avcodec/mpeg12dec: Do not call show_bits() with invalid bits

Fixes assertion failure
Fixes: 63e50545709a6440d3d59f6426d58db9/signal_sigabrt_7ffff6ae7cc9_8189_3272a3010fd98ddf947c662bbde1ac13.ts

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
