mjpegdec: Do not assume unused plane pointer are NULL.

We do neither document nor check such a requirement
and for application-provided get_buffer2 they could
contain the result of a malloc(0) or whatever value
they had previously.
This fixes a use-after-free in e.g. MPlayer:
https://trac.mplayerhq.hu/ticket/2262
We might want to consider changing the (documented)
API in addition though.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
