avcodec/dirac_parser: Check that there is a previous PU before accessing it

Fixes out of array read
Fixes: 99d142c47e6ba3510a74b872a1a2ae72/asan_heap-oob_11b36f4_3811_0f5c69e7609a88a580135678de1df844.dxa

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