ShellPkg: Fix file system change issue that results in ASSERT

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chris Phillips <chrisp@hp.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15406 6f19259b-4bc3-4df7-8a09-765794883524