MdeModulePkg/UdfDxe: Refine boundary checks for file/path name string

MdeModulePkg/UdfDxe: Refine boundary checks for file/path name string

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=828

The commit refines the boundary checks for file/path name string to
prevent possible buffer overrun.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jiewen Yao <jiewen.yao@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>
Acked-by: Star Zeng <star.zeng@intel.com>