avcodec/xpmdec: Move allocations down after more error checks

Fixes: Timeout
Fixes: 37035/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XPM_fuzzer-5142718576721920

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e5869283)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>