MdeModulePkg: fix warning about uninitialized variable

Handle.c:1302:24: error: 'Prot' may be used uninitialized
  in this function [-Werror=maybe-uninitialized]

       *Interface = Prot->Interface;
                    ~~~~^~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Mike Maslenkin <mike.maslenkin@gmail.com>

MdeModulePkg/Core/Dxe/Hand/Handle.c

@@ -1107,6 +1107,8 @@ CoreOpenProtocol (
     return EFI_INVALID_PARAMETER;
   }
 
+  Prot = NULL;
+
   //
   // Lock the protocol database
   //