GUI: ssrc_container: Zero initialize the new item

The remaining fields were set after the push_back(), but coverity
doesn't like that.

gui/QT/util/ssrc_container.hpp

@@ -31,7 +31,7 @@ inline void SSRC_container<T, Ts_type>::insert(uint32_t ssrc, const T& item, Ts_
 	}
 
 	if(!h){
-		Holder newItem;
+		Holder newItem = {};
 		newItem.ssrc = ssrc;
 		items.push_back(newItem);
 		h = &items.back();