fix area op exit with empty z list

src/kiri/app/init/input.js

@@ -75,6 +75,7 @@ function checkSeed(then) {
                 setTimeout(() => { api.space.save(true) },500);
                 then();
                 api.help.show();
+                sdb[LAST] = version;
             });
             return true;
         }

src/kiri/mode/cam/op-area.js

@@ -169,6 +169,8 @@ class OpArea extends CamOp {
                 let zinc = 1 / zs.length;
                 let lzo;
 
+                if (!zs.length) break;
+
                 outer: for (;;)
                 for (let z of zs) {
                     let slice = newLayer(z);