fdkaacenc: Fix draining with libfdk-aac v2.0.0
This release requires all buffer descriptor pointers to be valid, even when we provide zero input buffers.
Backport this one to 1.14, as a suspected fix for https://bugs.webkit.org/show_bug.cgi?id=197847 (can't be; this one is for the encoder)
Edited by Jan Alexander Steffens