user32: Use SecureZeroMemory to clear magic fields.

dlls/user32/dde_misc.c

@@ -2113,7 +2113,8 @@ void WDML_RemoveConv(WDML_CONV* pRef, WDML_SIDE side)
 	    {
 		pPrev->next = pCurrent->next;
 	    }
-	    pCurrent->magic = 0;
+            /* Ensure compiler doesn't optimize out the assignment with 0. */
+	    SecureZeroMemory(&pCurrent->magic, sizeof(pCurrent->magic));
 	    HeapFree(GetProcessHeap(), 0, pCurrent);
 	    break;
 	}