MdeModulePkg UdfDxe: Fix VS2010/VS2012 build failure

MdeModulePkg UdfDxe: Fix VS2010/VS2012 build failure

VS2010/VS2012 build failure with below info:
warning C4701:
  potentially uninitialized local variable 'DataOffset' used
  potentially uninitialized local variable 'FilePosition' used
  potentially uninitialized local variable 'FinishedSeeking' used
  potentially uninitialized local variable 'Data' used
warning C4703:
  potentially uninitialized local pointer variable 'Data' used

In fact, DataOffset, FilePosition and FinishedSeeking are initialized
and then used if (ReadFileInfo->Flags == READ_FILE_SEEK_AND_READ).
DoFreeAed will be set to TRUE when Data is allocated and returned from
GetAedAdsData(), and Data will be freed if (DoFreeAed) when exiting.

Use same method at 5afa5b815936e2b45a375b6521764195bed68680 to fix
the build failure.

There is related discussion at
https://lists.01.org/pipermail/edk2-devel/2017-September/014641.html

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Paulo Alcantara <pcacjr@zytor.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Paulo Alcantara <pcacjr@zytor.com>