avformat/rtsp: free the alloc memory if failed

Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

libavformat/rtsp.c

@@ -1245,8 +1245,10 @@ start:
         content = av_malloc(content_length + 1);
         if (!content)
             return AVERROR(ENOMEM);
-        if (ffurl_read_complete(rt->rtsp_hd, content, content_length) != content_length)
+        if (ffurl_read_complete(rt->rtsp_hd, content, content_length) != content_length) {
+            av_freep(&content);
             return AVERROR(EIO);
+        }
         content[content_length] = '\0';
     }
     if (content_ptr)