rtsp: Check a memory allocation

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtsp.c

@@ -1146,6 +1146,8 @@ start:
     if (content_length > 0) {
         /* leave some room for a trailing '\0' (useful for simple parsing) */
         content = av_malloc(content_length + 1);
+        if (!content)
+            return AVERROR(ENOMEM);
         ffurl_read_complete(rt->rtsp_hd, content, content_length);
         content[content_length] = '\0';
     }