Check InternalAllocPoolByIndex status before refer buffer.

Check InternalAllocPoolByIndex status before refer buffer.

Original code refers FreePoolHdr without check Status. It is obvious wrong and has risk.

Aslo, if InternalAllocPoolByIndex() returns an error, then *FreePoolHdr is assigned to an uninitialized value. So we init Hdr be NULL.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: "Zeng, Star" <star.zeng@intel.com>
Reviewed-by: "Yao, Jiewen" <jiewen.yao@intel.com>
Reviewed-by: "Fan, Jeff" <jeff.fan@intel.com>
Reviewed-by: "Kinney, Michael D" <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18932 6f19259b-4bc3-4df7-8a09-765794883524