FatPkg/FatPei/Gpt.c: Fix uninitialized variable issue

FatPkg/FatPei/Gpt.c: Fix uninitialized variable issue

Uninitialized pointer variable may randomly point to a block of memory.
In This case, FreePool function will free a block of memory that is not
belongs to this function.

Cc: Ruiyu Ni <ray.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>