BaseTools: Fix the crash issue when Dynamic structure Pcd use in FDF

BaseTools: Fix the crash issue when Dynamic structure Pcd use in FDF

The case is use Dynamic structure Pcd in the FDF file.
Current code already save the  Guid, Name and Filed info for those Pcd,
but it directly use the dict key as [Name, Guid] and cause this crash
issue.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=1264
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>