avformat/rmdec: check against mismatching int4 interleaver parameters which would leave uninitialized holes

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f9cf38857c0_4582_coop.ra
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/rmdec: move packet allocation down

Fixes memleak
Fixes: msan_uninit-mem_7fc5d73327d4_6192_kuerti.ra
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>