io/key,io/pointer: forbid nil tags for InputOps

Forcing a non-nil tag ensures that all handler tags are either unique,
or intentionally equal. Additionally, a nil tag has special meaning in
FocusOps.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/pointer/pointer.go

@@ -189,6 +189,9 @@ func (op CursorNameOp) Add(o *op.Ops) {
 }
 
 func (h InputOp) Add(o *op.Ops) {
+	if h.Tag == nil {
+		panic("Tag must be non-nil")
+	}
 	data := o.Write1(opconst.TypePointerInputLen, h.Tag)
 	data[0] = byte(opconst.TypePointerInput)
 	if h.Grab {