Qt Software Rendrer: Use a casted comparison instead of calling memcmp().

src/qt/qt_softwarerenderer.cpp

@@ -59,8 +59,7 @@ SoftwareRenderer::onBlit(int buf_idx, int x, int y, int w, int h)
 {
     /* TODO: should look into deleteLater() */
     auto  tval    = this;
-    void *nuldata = 0;
-    if (memcmp(&tval, &nuldata, sizeof(void *)) == 0)
+    if ((void *) tval == nullptr)
         return;
     auto origSource = source;