avcodec/mjpegenc: use av_freep(), avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegenc.c

@@ -78,7 +78,7 @@ av_cold int ff_mjpeg_encode_init(MpegEncContext *s)
 
 void ff_mjpeg_encode_close(MpegEncContext *s)
 {
-    av_free(s->mjpeg_ctx);
+    av_freep(&s->mjpeg_ctx);
 }
 
 static void encode_block(MpegEncContext *s, int16_t *block, int n)