Do not produce let as a variable name in mangle.

Would previously occur in large generated functions with 21,000+ variables.
Fixes #986.

test/mocha/let.js

@@ -0,0 +1,26 @@
+var Uglify = require('../../');
+var assert = require("assert");
+
+describe("let", function() {
+    it("Should not produce `let` as a variable name in mangle", function() {
+        // Produce a lot of variables in a function and run it through mangle.
+        var s = '"use strict"; function foo() {';
+        for (var i = 0; i < 21000; ++i) {
+            s += "var v" + i + "=0;";
+        }
+        s += '}';
+        var result = Uglify.minify(s, {fromString: true, compress: false});
+
+        // Verify that select keywords and reserved keywords not produced
+        assert.strictEqual(result.code.indexOf("var let="), -1);
+        assert.strictEqual(result.code.indexOf("var do="), -1);
+        assert.strictEqual(result.code.indexOf("var var="), -1);
+
+        // Verify that the variable names that appeared immediately before
+        // and after the erroneously generated `let` variable name still exist
+        // to show the test generated enough symbols.
+        assert(result.code.indexOf("var ket=") >= 0);
+        assert(result.code.indexOf("var met=") >= 0);
+    });
+});
+