obs-outputs: Fix FMS auth with query string

Fix FMS authentication to support RTMP connection URLs that have a
query string parameter already.

Closes jp9000/obs-studio#1105

plugins/obs-outputs/librtmp/rtmp.c

@@ -2526,8 +2526,10 @@ PublisherAuth(RTMP *r, AVal *description)
             }
             else if(r->Link.pubUser.av_len && r->Link.pubPasswd.av_len)
             {
+                const char *mark = r->Link.app.av_val ? strstr(r->Link.app.av_val, "?") : NULL;
                 pubToken.av_val = malloc(r->Link.pubUser.av_len + av_authmod_adobe.av_len + 8);
-                pubToken.av_len = sprintf(pubToken.av_val, "?%s&user=%s",
+                pubToken.av_len = sprintf(pubToken.av_val, "%s%s&user=%s",
+                                          mark != NULL ? "&" : "?",
                                           av_authmod_adobe.av_val,
                                           r->Link.pubUser.av_val);
                 RTMP_Log(RTMP_LOGDEBUG, "%s, pubToken1: %s", __FUNCTION__, pubToken.av_val);