remove context unmatched warning in QOpenGLTexture

destroy() or destructor complain when QOpenGLTexture is not created or
it is already destroyed.

Change-Id: I6b3135849e3ba2ce35678fcdbf1c9b6e588a063c
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/gui/opengl/qopengltexture.cpp

@@ -180,6 +180,10 @@ bool QOpenGLTexturePrivate::create()
 
 void QOpenGLTexturePrivate::destroy()
 {
+    if (!context) {
+        // not created or already destroyed
+        return;
+    }
     if (QOpenGLContext::currentContext() != context) {
         qWarning("Requires a valid current OpenGL context.\n"
                  "Texture has not been destroyed");