MdeModulePkg DxeCore: Only free ScratchBuffer when it is not NULL

MdeModulePkg DxeCore: Only free ScratchBuffer when it is not NULL

There is a case that ExtractGuidedSectionGetInfo return 0 for
ScratchBufferSize and ScratchBuffer will be NULL, after AllocatePool
fails to allocate buffer for AllocatedOutputBuffer, the code will
call FreePool (ScratchBuffer), but ScratchBuffer == NULL.

This patch is to only free ScratchBuffer when it is not NULL.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>