fix regression by selecting clipboard text

"restore the old behaviour that the primary doesn't get deleted by a simple
left click"

Patch by Daniel Tameling <tamelingdaniel@gmail.com>, thanks!

x.c

@@ -618,6 +618,9 @@ selrequest(XEvent *e)
 void
 setsel(char *str, Time t)
 {
+	if (!str)
+		return;
+
 	free(xsel.primary);
 	xsel.primary = str;