1) Add type cast for better coding style.

1) Add type cast for better coding style.
2) replace StrCpy() usage in Variable driver with StrnCpy().

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15770 6f19259b-4bc3-4df7-8a09-765794883524