librtmp: append the correct field to the string

Also prevent a NULL pointer dereference.

CC: libav-stable@libav.org
Bug-Id: CID 1250329 / CID 1250331
(cherry picked from commit a28468d0daf4be14761c16a3ddd33266b2380123)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/librtmp.c

@@ -169,7 +169,7 @@ static int rtmp_open(URLContext *s, const char *uri, int flags)
     }
     if (ctx->swfurl) {
         av_strlcat(filename, " swfUrl=", len);
-        av_strlcat(filename, ctx->pageurl, len);
+        av_strlcat(filename, ctx->swfurl, len);
     }
     if (ctx->flashver) {
         av_strlcat(filename, " flashVer=", len);