Add PubKey index check before touching PubKey cache. Internal PubKey Variable PubkeyI...

Add PubKey index check before touching PubKey cache. Internal PubKey Variable PubkeyIndex is always 0, causing Index – 1 overflow. Update corresponding comments.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chao, Zhang <chao.b.zhang@intel.com>
Reviewed-by: Dong, Guo <guo.dong@intel.com>
Reviewed-by: Fu, Siyuan <siyuan.fu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16220 6f19259b-4bc3-4df7-8a09-765794883524