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 467d9e27e0.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mjpegenc.c

@@ -312,8 +312,10 @@ av_cold int ff_mjpeg_encode_init(MpegEncContext *s)
 
 av_cold void ff_mjpeg_encode_close(MpegEncContext *s)
 {
-    av_freep(&s->mjpeg_ctx->huff_buffer);
-    av_freep(&s->mjpeg_ctx);
+    if (s->mjpeg_ctx) {
+        av_freep(&s->mjpeg_ctx->huff_buffer);
+        av_freep(&s->mjpeg_ctx);
+    }
 }
 
 /**