Nt32Pkg/WinNtSimpleFileSystemDxe: Fix ASSERT() parsing '\'

https://bugzilla.tianocore.org/show_bug.cgi?id=331

If Nt32 is built using UEFI Shell from the ShellPkg sources,
an ASSERT() is generated when a single '\' character is
entered at the shell prompt.

The WinNtSimpleFileSystemDxe module GetNextFileNameToken()
function breaks a file path up into tokens, but it does not
handle the case where a FileName ends in a '\' character.
It returns an empty string instead of NULL.  The fix is
to set *FileName to NULL if the remaining file path is an
empty string.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

Nt32Pkg/WinNtSimpleFileSystemDxe/WinNtSimpleFileSystem.c

@@ -1,6 +1,6 @@
 /**@file
 
-Copyright (c) 2006 - 2016, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
 which accompanies this distribution.  The full text of the license may be found at
@@ -748,6 +748,12 @@ GetNextFileNameToken (
     // Point *FileName to the next character after L'\'.
     //
     *FileName = *FileName + Offset + 1;
+    //
+    // If *FileName is an empty string, then set *FileName to NULL
+    //
+    if (**FileName == L'\0') {
+      *FileName = NULL;
+    }
   }
 
   return Token;