QOpenGLTexture: check textureId for knowing whether a texture was created

Change-Id: I0775ad9538a7793dc6628abe4556404634ae0462
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/gui/opengl/qopengltexture.cpp

@@ -175,7 +175,7 @@ bool QOpenGLTexturePrivate::create()
 
 void QOpenGLTexturePrivate::destroy()
 {
-    if (!context) {
+    if (!textureId) {
         // not created or already destroyed
         return;
     }