avformat/rtsp: check memory allocation

Reviewed-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Commit message by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtsp.c

@@ -1150,6 +1150,8 @@ 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';
     }