avcodec/mjpegenc: Fix segfault when freeing incomplete context
When allocating the MJpegContext fails (or if the dimensions run afoul
of the 65500x65500 limit), an attempt to free a subbuffer of said
context leads to a segfault in ff_mjpeg_encode_close().
Seems to be a regression since
467d9e27e0cb2bf74f41dc832f2f8d191ba58ec9.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit
84ac35ecb8a53bf313b468b5a9f1b0617f2a3de2)