dtls: Check if pad stopped while sending data

If sending data would block, we can get into an infinite loop retrying while holding the stream lock. Check to see if the pad as been deactivated and bail.

Fixes a timeout I was occasionally seeing in the webrtcbin tests.

ext/dtls/gstdtlsenc.c

@@ -565,6 +565,9 @@ sink_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
         GST_ELEMENT_ERROR (self, RESOURCE, WRITE, (NULL), ("%s", err->message));
         g_clear_error (&err);
         break;
+      case GST_FLOW_FLUSHING:
+        GST_INFO_OBJECT (self, "Flushing");
+        break;
       default:
         g_assert_not_reached ();
         break;
@@ -680,6 +683,8 @@ on_send_data (GstDtlsConnection * connection, gconstpointer data, gsize length,
   GST_TRACE_OBJECT (self, "send data: releasing lock");
 
   ret = self->src_ret == GST_FLOW_OK;
+  if (self->src_ret == GST_FLOW_FLUSHING)
+    gst_dtls_connection_set_flow_return (connection, self->src_ret);
   g_mutex_unlock (&self->queue_lock);
 
   return ret;