Fix potential call of uninitialized pointer function

_template_number can be set via sysex to an arbitrary number, this
can lead to calling a filter-function at an undefined address, usually
a segfault.

libs/surfaces/launch_control_xl/launch_control_xl.cc

@@ -1002,6 +1002,8 @@ LaunchControlXL::filter_stripables(StripableList& strips) const
 	FilterFunction flt;
 
 	switch ((int)template_number()) {
+		default:
+			/* FALLTHROUGH */
 		case 8:
 			flt = &flt_default;
 			break;