rtpsession: Add missing lock around sess->ssrcs iteration

https://bugzilla.gnome.org/show_bug.cgi?id=795139
diff --git a/gst/rtpmanager/rtpsession.c b/gst/rtpmanager/rtpsession.c
index 0b99d23..ff0c09a 100644
--- a/gst/rtpmanager/rtpsession.c
+++ b/gst/rtpmanager/rtpsession.c
@@ -3973,11 +3973,15 @@
   GHashTableIter iter;
   RTPSource *src;
 
+  RTP_SESSION_LOCK (sess);
   g_hash_table_iter_init (&iter, sess->ssrcs[sess->mask_idx]);
   while (g_hash_table_iter_next (&iter, NULL, (gpointer *) & src)) {
-    if (src->internal && !src->sent_bye)
+    if (src->internal && !src->sent_bye) {
+      RTP_SESSION_UNLOCK (sess);
       return FALSE;
+    }
   }
+  RTP_SESSION_UNLOCK (sess);
 
   return TRUE;
 }