Fix bug introduced by r12827.

If av_exit() was called after an AVFormatContext was created, but before
its file was opened, av_exit() would pass a NULL pointer to url_fclose().

Originally committed as revision 12915 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -380,7 +380,7 @@ static int av_exit(int ret)
         /* maybe av_close_output_file ??? */
         AVFormatContext *s = output_files[i];
         int j;
-        if (!(s->oformat->flags & AVFMT_NOFILE))
+        if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
             url_fclose(s->pb);
         for(j=0;j<s->nb_streams;j++) {
             av_free(s->streams[j]->codec);