vcodec/rv34: use av_mallocz() to allocate tables

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f75e2a55b88_4146_brokenaudio.rmvb
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>