avcodec/h264_slice: replace assert by normal error check

Fixes assertion failure
Fixes: c6075771557e4f3b7b74e63d2d24fb01/signal_sigabrt_7ffff6ac8cc9_133_cov_2853689970_CREDITS.FST

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