MdeModulePkg/UdfDxe: ASSERT for false positives of NULL ptr deref

MdeModulePkg/UdfDxe: ASSERT for false positives of NULL ptr deref

This commit adds ASSERTs to address false positive reports of NULL
pointer dereference issues raised from static analysis with regard to
function ReadDirectoryEntry().

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>
Reviewed-by: Star Zeng <star.zeng@intel.com>