avcodec/mjpegdec: Dont skip picture allocation if theres no picture allocated

Fixes Ticket 3245
(cherry picked from commit ad8d063f230c05f8b5efbd05cc5a9f51a2549dcf)
Allow decoding of slightly broken Nikon avi files.

Fixes ticket #3330.
(cherry picked from commit f9c2d4d17e3b18becb046d71811f9e8aa5946cf9)

Conflicts:
	libavformat/avidec.c