[NTUSER] Lock before getting DesktopDC. Fixes crash during loading DirectX CORE-7733

svn path=/trunk/; revision=74142

reactos/win32ss/user/ntuser/desktop.c

@@ -932,6 +932,8 @@ UserGetDesktopDC(ULONG DcType, BOOL EmptyDC, BOOL ValidatehWnd)
     PWND DesktopObject = 0;
     HDC DesktopHDC = 0;
 
+    UserEnterExclusive();
+
     if (DcType == DC_TYPE_DIRECT)
     {
         DesktopObject = UserGetDesktopWindow();
@@ -943,6 +945,8 @@ UserGetDesktopDC(ULONG DcType, BOOL EmptyDC, BOOL ValidatehWnd)
         DesktopHDC = IntGdiCreateDisplayDC(pMonitor->hDev, DcType, EmptyDC);
     }
 
+    UserLeave();
+
     return DesktopHDC;
 }