rtsp: Check for command strings without spaces

Prevent a NULL-pointer dereference.

CC: libav-stable@libav.org

libavformat/rtspdec.c

@@ -360,6 +360,10 @@ static inline int parse_command_line(AVFormatContext *s, const char *line,
     RTSPState *rt = s->priv_data;
     const char *linept, *searchlinept;
     linept = strchr(line, ' ');
+
+    if (!linept)
+        return AVERROR_INVALIDDATA;
+
     if (linept - line > methodsize - 1) {
         av_log(s, AV_LOG_ERROR, "Method string too long\n");
         return AVERROR(EIO);