Ensure group is unescaped properly in kconf_update.

During a kconf_update run, an invalid group name may be treated as correct
even though the name failed to unescape.  This leads the group name to be
a null character, which will fail.

If the unescape failed, return that failure status instead.  This should have
no impact, as the previous result would have been wrong anyways.  Now a
more useful diagnostic will be returned instead.

Update the unit tests to ensure this issue is tested in the future.

Found in Coverity issue 258087.

REVIEW: 126556

autotests/test_kconfigutils.cpp

@@ -57,6 +57,12 @@ void TestKConfigUtils::testParseGroupString_data()
             << QStringList()
             << false
             ;
+
+    QTest::newRow("invalid-escape-string")
+            << "[a\\z]"
+            << QStringList()
+            << false
+            ;
 }
 
 void TestKConfigUtils::testParseGroupString()

src/kconf_update/kconfigutils.cpp

@@ -46,7 +46,7 @@ KConfigGroup openGroup(KConfig *config, const QStringList &lst)
 QStringList parseGroupString(const QString &_str, bool *ok, QString *error)
 {
     QString str = unescapeString(_str.trimmed(), ok, error);
-    if (!ok) {
+    if (!*ok) {
         return QStringList();
     }