avcodec/mjpegdec: fix SOF check in EOI

avcodec/mjpegdec: fix SOF check in EOI

For frames decoded with skip_frame == AVDISCARD_ALL, a picture is not allocated
and got_picture is never set to 1 even if a SOF and SOS were parsed.
The existing check in EOI only cares if a SOF was parsed, not if a picture
allocated, so change it and add a new check to explicitly ensure a picture was
allocated when skip_frame != AVDISCARD_ALL.

Fixes probing and decoding when skip_frame is AVDISCARD_ALL.

Signed-off-by: James Almer <jamrial@gmail.com>