fdkaacenc: fix accessing freed memory

fdkaacenc: fix accessing freed memory

The buffer data is not always copied in _Fill, and will be
read in _DecodeFrame. We unmap at the end of the function,
whether we get there via failure or early out, and keep a
ref to the buffer to ensure we can use it to unmap the
memory even after _finish_frame is called, as it unrefs
the buffer.

Note that there is an access beyond the allocated buffer,
which is only apparent when playing from souphttpsrc (ie,
not from filesrc). This appears to be a bug in the bit
reading code in libfdkaac AFAICT.

https://bugzilla.gnome.org/show_bug.cgi?id=772186