avcodec/alac: Clear pointers in allocate_buffers()

      Fixes: 06a4edb39ad8a9883175f9bd428334a2_signal_sigsegv_7ffff713351a_706_mov__alac__ALAC_6ch.mov

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