avformat/rtsp: Fix null pointer dereference

This fixes CID 1257827.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtsp.c

@@ -2259,6 +2259,8 @@ static int sdp_read_header(AVFormatContext *s)
     /* read the whole sdp file */
     /* XXX: better loading */
     content = av_malloc(SDP_MAX_SIZE);
+    if (!content)
+        return AVERROR(ENOMEM);
     size = avio_read(s->pb, content, SDP_MAX_SIZE - 1);
     if (size <= 0) {
         av_free(content);