avcodec/truemotion1: Initialize mb_change_byte only when needed

Fixes out of array read
Fixes: d92114d8c2a019b8a6e50cd2a7301b54/asan_heap-oob_26bf563_60_1d3420277533de9dbf8aba3f93af346f.avi

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