LibJS: Check validity of computed_property_name() result before using it

This fixes two cases obj[expr] and obj[expr]() (MemberExpression and
CallExpression respectively) when expr throws an exception and results
in an empty value, causing a crash by passing the invalid PropertyName
created by computed_property_name() to Object::get() without checking it
first.

Fixes #3459.

Libraries/LibJS/Tests/computed-property-throws.js

@@ -0,0 +1,8 @@
+test("Issue #3459, exception in computed property expression", () => {
+    expect(() => {
+        "foo"[bar];
+    }).toThrow(ReferenceError);
+    expect(() => {
+        "foo"[bar]();
+    }).toThrow(ReferenceError);
+});