avconv: check that the output format context exists before accessing it

Fixes a segfault in exit_program() if opening an output file fails.

avconv.c

@@ -167,7 +167,7 @@ static void exit_program(void)
     /* close files */
     for (i = 0; i < nb_output_files; i++) {
         AVFormatContext *s = output_files[i]->ctx;
-        if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
+        if (s && s->oformat && !(s->oformat->flags & AVFMT_NOFILE) && s->pb)
             avio_close(s->pb);
         avformat_free_context(s);
         av_dict_free(&output_files[i]->opts);