Don't crash when converting missing array elements to assignable

Closes #220

test/tests-harmony.js

@@ -15466,6 +15466,34 @@ test("let {x} = y", {
   "end": 11
 }, {ecmaVersion: 6})
 
+test("[x,,] = 1", {
+  type: "Program",
+  body: [
+    {
+      type: "ExpressionStatement",
+      expression: {
+        type: "AssignmentExpression",
+        operator: "=",
+        left: {
+          type: "ArrayPattern",
+          elements: [
+            {
+              type: "Identifier",
+              name: "x"
+            },
+            null
+          ]
+        },
+        right: {
+          type: "Literal",
+          value: 1,
+          raw: "1"
+        }
+      }
+    }
+  ]
+}, {ecmaVersion: 6});
+
 testFail("let [x]", "Complex binding patterns require an initialization value (1:7)", {ecmaVersion: 6})
 testFail("var [x]", "Complex binding patterns require an initialization value (1:7)", {ecmaVersion: 6})
 testFail("var _𖫵 = 11;", "Unexpected character '𖫵' (1:5)", {ecmaVersion: 6});