vis-lua: fail when mapping a key to an invalid handler type

When passing an invalid handler type (i.e., any type that isn't a
string, function, or KeyAction) to Vis:map/Window:map, the editor would
map the key to an empty (zeroed) KeyBinding struct.  vis_keys_process()
doesn't take this into account, so the key is never consumed from
the input queue, causing the editor to get stuck in an infinite loop.

vis-lua.c

@@ -832,6 +832,8 @@ static int keymap(lua_State *L, Vis *vis, Win *win) {
 			goto err;
 	} else if (lua_isuserdata(L, 4)) {
 		binding->action = obj_ref_check(L, 4, VIS_LUA_TYPE_KEYACTION);
+	} else {
+		goto err;
 	}
 
 	if (win) {