rtmpproto: send swfverify value as swfurl if latter is unused

Replicates lavf/librtmp.c behavior in L149-156 and rtmpdump's
behavior with "--swfVfy <url>" passing the url to swfUrl.

Fixes trac ticket #5549.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rtmpproto.c

@@ -341,9 +341,12 @@ static int gen_connect(URLContext *s, RTMPContext *rt)
     ff_amf_write_field_name(&p, "flashVer");
     ff_amf_write_string(&p, rt->flashver);
 
-    if (rt->swfurl) {
+    if (rt->swfurl || rt->swfverify) {
         ff_amf_write_field_name(&p, "swfUrl");
-        ff_amf_write_string(&p, rt->swfurl);
+        if (rt->swfurl)
+            ff_amf_write_string(&p, rt->swfurl);
+        else
+            ff_amf_write_string(&p, rt->swfverify);
     }
 
     ff_amf_write_field_name(&p, "tcUrl");