avcodec/xpmdec: Fix multiple pointer/memory issues

Most of these were found through code review in response to
fixing 1466/clusterfuzz-testcase-minimized-5961584419536896
There is thus no testcase for most of this.
The initial issue was Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg

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