LibJS: Don't assume match for each capture group in RegExp.prototype.exec()

This was not implementing the following part of the spec correctly:

    27. For each integer i such that i ≥ 1 and i ≤ n, do
        a. Let captureI be ith element of r's captures List.
        b. If captureI is undefined, let capturedValue be undefined.

Expecting a capture group match to exist for each of the RegExp's
capture groups would assert in Vector's operator[] if that's not the
case, for example:

    /(foo)(bar)?/.exec("foo")

Append undefined instead.

Fixes #5256.

Userland/Libraries/LibJS/Runtime/RegExpPrototype.cpp

@@ -190,8 +190,12 @@ JS_DEFINE_NATIVE_FUNCTION(RegExpPrototype::exec)
     array->indexed_properties().put(array, 0, js_string(vm, match.view.to_string()));
 
     for (size_t i = 0; i < result.n_capture_groups; ++i) {
-        auto& capture = result.capture_group_matches[0][i];
-        array->indexed_properties().put(array, i + 1, js_string(vm, capture.view.to_string()));
+        auto capture_value = js_undefined();
+        if (result.capture_group_matches[0].size() > i) {
+            auto& capture = result.capture_group_matches[0][i];
+            capture_value = js_string(vm, capture.view.to_string());
+        }
+        array->indexed_properties().put(array, i + 1, capture_value);
     }
 
     Value groups = js_undefined();

Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.exec.js

@@ -18,6 +18,16 @@ test("basic unnamed captures", () => {
     expect(res[1]).toBe("ooooo");
     expect(res.groups).toBe(undefined);
     expect(res.index).toBe(0);
+
+    re = /(foo)(bar)?/;
+    res = re.exec("foo");
+
+    expect(res.length).toBe(3);
+    expect(res[0]).toBe("foo");
+    expect(res[1]).toBe("foo");
+    expect(res[2]).toBe(undefined);
+    expect(res.groups).toBe(undefined);
+    expect(res.index).toBe(0);
 });
 
 test("basic named captures", () => {