avcodec/qdrw: Fix null pointer dereference

The RGB555 PACKBITSRGN case tries to read a palette, if such
palette is actually stored then it accesses a null pointer.
All 16bit samples i could find use DIRECTBITSRGN.

Fixes: 2065/clusterfuzz-testcase-minimized-6298930457346048

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
