avformat/mux: Check that deinit is set before calling it

Fixes null pointer dereference
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mux.c

@@ -424,6 +424,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
     }
 
     if (s->oformat->init && (ret = s->oformat->init(s)) < 0) {
+        if (s->oformat->deinit)
             s->oformat->deinit(s);
         goto fail;
     }