Fix JS compiler crash when translating external object fun with vararg

See KT-19793

js/js.translator/testData/box/native/objectFunWithVararg.js

@@ -0,0 +1,9 @@
+Test = {
+    test: function() {
+        var sum = 0;
+        for (var i = 0; i < arguments.length; ++i) {
+            sum += arguments[i];
+        }
+        return sum;
+    }
+};

js/js.translator/testData/box/native/objectFunWithVararg.kt

@@ -0,0 +1,13 @@
+// EXPECTED_REACHABLE_NODES: 995
+import Test.test
+
+external object Test {
+    fun test(vararg rest: Int): Int
+}
+
+fun box(): String {
+    val result = test(23, 42)
+    if (result != 65) return "fail: $result"
+
+    return "OK"
+}