avformat/rmdec: check that sub_packet_size fit in the packet for DEINT_ID_GENR

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fa8c49400d0_3923_audiosig.rm
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
