MdeModulePkg EhciPei: Also check Buf against NULL to return error

Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>

MdeModulePkg/Bus/Pci/EhciPei/EhciSched.c

@@ -134,7 +134,7 @@ EhcInitSched (
              &Map
              );
 
-  if (EFI_ERROR (Status)) {
+  if (EFI_ERROR (Status) || (Buf == NULL)) {
     return EFI_OUT_OF_RESOURCES;
   }