Explicitly do nothing when an empty container is passed to delete_clear as it's oddly slow otherwise

Originally committed to SVN as r6402.

aegisub/libaegisub/include/libaegisub/util.h

@@ -74,9 +74,11 @@ namespace agi {
 	};
 	template<class T>
 	void delete_clear(T& container) {
+		if (!container.empty()) {
 			std::for_each(container.begin(), container.end(), delete_ptr());
 			container.clear();
 		}
+	}
 
 	} // namespace util
 } // namespace agi