avcodec/diracdec: Check for hpel_base allocation failure

Fixes null pointer dereference
Fixes: signal_sigsegv_b02a96_280_RL_420p_ffdirac.drc with memlimit of 67108864

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
