MdeModulePkg/DpcLib: return error if failed to locate DPC protocol.

This patch updates the constructor of DpcLib to return error if failed to
locate DPC protocol.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>

MdeModulePkg/Library/DxeDpcLib/DpcLib.c

@@ -45,7 +45,7 @@ DpcLibConstructor (
   Status = gBS->LocateProtocol (&gEfiDpcProtocolGuid, NULL, (VOID **)&mDpc);
   ASSERT_EFI_ERROR (Status);
 
-  return EFI_SUCCESS;
+  return Status;
 }
 
 /**