check if stream is set, fix potential segv

Originally committed as revision 15693 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffserver.c

@@ -4018,11 +4018,13 @@ static int parse_ffconfig(const char *filename)
             }
         } else if (!strcasecmp(cmd, "InputFormat")) {
             get_arg(arg, sizeof(arg), &p);
+            if (stream) {
             stream->ifmt = av_find_input_format(arg);
             if (!stream->ifmt) {
                 fprintf(stderr, "%s:%d: Unknown input format: %s\n",
                         filename, line_num, arg);
             }
+            }
         } else if (!strcasecmp(cmd, "FaviconURL")) {
             if (stream && stream->stream_type == STREAM_TYPE_STATUS) {
                 get_arg(stream->feed_filename, sizeof(stream->feed_filename), &p);