MdeModulePkg Variable: Move CacheVariable->InDeletedTransitionPtr NULL pointer check

MdeModulePkg Variable: Move CacheVariable->InDeletedTransitionPtr NULL pointer check

This change is triggered by R18654 "Enhance variable performance by reading from existed memory cache"
that changed the code to use CacheVariable to improve performance.
So the CacheVariable->InDeletedTransitionPtr NULL pointer check needs to be moved to the place before
it is been used.

Cc: Shumin Qiu <shumin.qiu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Shumin Qiu <shumin.qiu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18688 6f19259b-4bc3-4df7-8a09-765794883524