MdeModulePkg/Core: fix guard page missing issue

MdeModulePkg/Core: fix guard page missing issue

This issue is a regression one caused by a patch at

    425d25699be83c35e12df8470b827d7fbcef3bce

That fix didn't take the 0 page to free into account, which still
needs to call UnsetGuardPage() even no memory needs to free.

The fix is just moving the calling of UnsetGuardPage() to the place
right after calling AdjustMemoryF().

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>