avcodec/msmpeg4dec: initialize dir_ptr in error cases

Fixes use of uinitialized memory
Fixes: msan_uninit-mem_7f01b8799186_7278_msnaudio.asf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
