Delete the d_ptr in the destructor

The d_ptr variable is introduced by QGLCustomShaderStage and
needs to be fully handled by it. Delete the d_ptr in the dtor

Fixes Coverity CID #10995.

Change-Id: I76bf05206acf23363419afc95094f5593e808a2e
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/opengl/gl2paintengineex/qglcustomshaderstage.cpp

@@ -63,6 +63,7 @@ QGLCustomShaderStage::~QGLCustomShaderStage()
         d->m_manager->removeCustomStage();
         d->m_manager->sharedShaders->cleanupCustomStage(this);
     }
+    delete d_ptr;
 }
 
 void QGLCustomShaderStage::setUniformsDirty()