avcodec/mpegvideo_enc: Check for integer overflow in ff_mpv_reallocate_putbitbuffer()

Fixes assertion failure
Fixes: 6568d187979ce17878b6fe5fbbb89142/signal_sigabrt_7ffff6ae7cb7_7176_564bbc6741bdcf907f5c4e685c9a77a2.mpg

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