win-capture: Always set class/title/exe to null

When getting the class/title/exe of a particular window handle in the
build_window_strings function, always set the class/title/exe pointers
to null to prevent any potential references to invalid values if any of
them do not happen to be set for whatever reason.

plugins/win-capture/window-helpers.c

@@ -29,10 +29,11 @@ extern void build_window_strings(const char *str,
 {
 	char **strlist;
 
+	*class = NULL;
+	*title = NULL;
+	*exe   = NULL;
+
 	if (!str) {
-		*class = NULL;
-		*title = NULL;
-		*exe   = NULL;
 		return;
 	}