MdeModulePkg/FvSimpleFileSystem:Fix a potential NULL dereference issue.

Contributed-under: TianoCore Contribution Agreement 1.0

Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Shumin Qiu <shumin.qiu@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16562 6f19259b-4bc3-4df7-8a09-765794883524