avcodec/hevc: use av_mallocz() for allocating tab_ipm

Fixes use of uninitialized memory and out of stack array read
Fixes: signal_sigsegv_ecc526_7846_WPP_C_ericsson_MAIN_2.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
