postproc: fix unaligned access

QP_store is only 8-bit-aligned, so accessing it as uint32_t causes
SIGBUS crashes on sparc.
The AV_RN32/AV_WN32 macros only do unaligned access in the
HAVE_FAST_UNALIGNED case.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
