rtmp: fix url parsing

The application component can have a subcomponent to specify the
application instance even if it doesn't have a ":" in the playpath.

doc/protocols.texi

@@ -164,7 +164,7 @@ content across a TCP/IP network.
 
 The required syntax is:
 @example
-rtmp://@var{server}[:@var{port}][/@var{app}][/@var{playpath}]
+rtmp://@var{server}[:@var{port}][/@var{app}][/@var{instance}][/@var{playpath}]
 @end example
 
 The accepted parameters are:

libavformat/rtmpproto.c

@@ -1037,9 +1037,10 @@ static int rtmp_open(URLContext *s, const char *uri, int flags)
             fname = next;
             rt->app[0] = '\0';
         } else {
+            // make sure we do not mismatch a playpath for an application instance
             char *c = strchr(p + 1, ':');
             fname = strchr(p + 1, '/');
-            if (!fname || c < fname) {
+            if (!fname || (c && c < fname)) {
                 fname = p + 1;
                 av_strlcpy(rt->app, path + 1, p - path);
             } else {