LibJS: Check for exception after executing (do)while test expression

Otherwise we crash the interpreter when an exception is thrown during
evaluation of the while or do/while test expression - which is easily
caused by a ReferenceError - e.g.:

    while (someUndefinedVariable) {
        // ...
    }

Libraries/LibJS/Tests/loops/while-basic.js

@@ -17,3 +17,9 @@ test("does not loop when initially false", () => {
         expect().fail();
     }
 });
+
+test("exception in test expression", () => {
+    expect(() => {
+        while (foo);
+    }).toThrow(ReferenceError);
+});