Gui: fix memory leak in QGuiGLThreadContext

`QCoreApplicationPrivate::cleanupThreadData` calls
`~QGuiGLThreadContext`, which calls
`QOpenGLContextPrivate::setCurrentContext`, which creates a new
`QGuiGLThreadContext`, which is not destroyed anymore.

since `~QGuiGLThreadContext` sets a nullptr we exit early.

Fixes: QTBUG-124538
Pick-to: 6.7
Change-Id: I51e40fcf8fd1169a4dfd336fac9c82f44d42f68e
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/kernel/qopenglcontext.cpp

@@ -170,6 +170,9 @@ QOpenGLContext *QOpenGLContextPrivate::setCurrentContext(QOpenGLContext *context
             qWarning("No QTLS available. currentContext won't work");
             return nullptr;
         }
+        if (!context)
+            return nullptr;
+
         threadContext = new QGuiGLThreadContext;
         qwindow_context_storage()->setLocalData(threadContext);
     }