avcodec/mlpdec: check that streamtype matches the codec

Fixes use of uninitialized memory
Fixes msan_uninit-mem_7f126c8ed1ac_5945_issue1731_nonpublic.mpg
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>