Fixes: NULL pointer dereference
Fixes: 26814/clusterfuzz-testcase-minimized-ffmpeg_dem_ICO_fuzzer-5758487797432320

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