keypress, check if len > 0 just in case before reading buf

I'm not sure if this can happen, but check it just in case:

Maybe possible when:

len = XmbLookupString(xic, ev, buf, sizeof buf, &ksym, &status);
len == 0 and case XLookupChars would be executed on a keypress.

Reported by: jb19357 <jb19357@protonmail.com>
Who used Claude to write a report.

I cannot reproduce this issue, but it seems logical.

dmenu.c

@@ -414,7 +414,7 @@ keypress(XKeyEvent *ev)
 	switch(ksym) {
 	default:
 insert:
-		if (!iscntrl((unsigned char)*buf))
+		if (len > 0 && !iscntrl((unsigned char)*buf))
 			insert(buf, len);
 		break;
 	case XK_Delete: