avcodec/interplayvideo: Check ff_get_buffer() for failure

Fixes: runtime error: division by zero
Fixes: 2408/clusterfuzz-testcase-minimized-5432734438653952
Fixes: 2415/clusterfuzz-testcase-minimized-4672827619803136

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