use http_log when log file is setup

Originally committed as revision 13847 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffserver.c

@@ -4504,7 +4504,7 @@ int main(int argc, char **argv)
     }
 
     if (http_server() < 0) {
-        fprintf(stderr, "Could not start server\n");
+        http_log("Could not start server\n");
         exit(1);
     }