ext/ffmpeg/gstffmpegdec.c: Don't use gst_pad_alloc_buffer() for frames with palette, as we'll artificially shorten th...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_frame):
Don't use gst_pad_alloc_buffer() for frames with palette,
as we'll artificially shorten the size of buffers in that
case and GstBaseTransform will complain about unit size
issues if we use _alloc_buffer() with the full size.