MdeModulePkg/UdfDxe: Avoid possible use of already-freed data

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1255

For function ReadFile():

If the line

  Status = GetAedAdsData (
   ...
   );

is reached multiple times during the 'for' loop, freeing the data pointed
by variable 'Data' may potentially lead to variable 'Ad' referencing the
already-freed data.

After calling function GetAllocationDescriptor(), 'Data' and 'Ad' may
point to the same memory (with some possible offset). Hence, this commit
will move the FreePool() call backwards to ensure the data will no longer
be used.

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>
Acked-by: Star Zeng <star.zeng@intel.com>