The size 0 special case causes side data to be created which is
different and a special case if for any reasons size = 0 is passed

Fixes: multiple runtime error: null pointer passed as argument 1, which is declared to never be null
Fixes: 653/clusterfuzz-testcase-5773837415219200

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