avcodec/rv34: Clear pointers in ff_rv34_decode_init_thread_copy()

      Avoids leaving stale pointers
Fixes: signal_sigabrt_7ffff70eccc9_819_sabtriple.rm with memlimit 536870912

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3197c0aa87a3b7190e17d49e6fbc7b554e4b3f0a)

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>