MdeModulePkg/UdfDxe: Handle dead codes in File.c

MdeModulePkg/UdfDxe: Handle dead codes in File.c

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1249

We found potential dead codes within File.c during the code coverage test.

After manual review, we think the below ones are positive reports:

A. In function UdfRead():
  else if (IS_FID_DELETED_FILE (Parent->FileIdentifierDesc)) {
    Status = EFI_DEVICE_ERROR;
  }

A File Identifier Descriptor will be get from the UDF media only by
function ReadDirectoryEntry(). And within this function, all the File
Identifier Descriptor with 'DELETED_FILE' characteristics will be skipped
and will not be returned. Hence, the above codes in function UdfRead()
will never be hit.

This commit will add "ASSERT (FALSE);" before the above line to indicate
this.

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>