MdeModulePkg/UdfDxe: Use debug msg instead of ASSERT in UdfOpen()

MdeModulePkg/UdfDxe: Use debug msg instead of ASSERT in UdfOpen()

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1248

Within function UdfOpen():
This commit will use debug messages instead of using ASSERT when an error
occurs after calling GetFileSize().

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>