use xcb_connection_has_error to check for failue

xcb_connectxx() always returns a non-NULL pointer to a
xcb_connection_t, even on failure.

src/backends/x11/windowed/x11_windowed_backend.cpp

@@ -172,7 +172,8 @@ X11WindowedBackend::~X11WindowedBackend()
 bool X11WindowedBackend::initialize()
 {
     m_connection = xcb_connect(m_options.display.toLatin1(), &m_screenNumber);
-    if (!m_connection) {
+    if (xcb_connection_has_error(m_connection)) {
+        xcb_disconnect(m_connection);
         return false;
     }
 

tests/normalhintsbasesizetest.cpp

@@ -34,6 +34,11 @@ int main(int, char **)
     int screenNumber;
     xcb_connection_t *c = xcb_connect(nullptr, &screenNumber);
 
+    if (xcb_connection_has_error(c)) {
+        xcb_disconnect(c);
+        return 1;
+    }
+
     auto getScreen = [=]() {
         const xcb_setup_t *setup = xcb_get_setup(c);
         auto it = xcb_setup_roots_iterator(setup);

tests/pointerconstraintstest.cpp

@@ -240,8 +240,10 @@ void XBackend::init(QQuickView *view)
 {
     Backend::init(view);
     m_xcbConn = xcb_connect(nullptr, nullptr);
-    if (!m_xcbConn) {
-        qDebug() << "Could not open XCB connection.";
+
+    if (xcb_connection_has_error(m_xcbConn)) {
+        xcb_disconnect(m_xcbConn);
+        qFatal() << "Could not open XCB connection.";
     }
 }