MdePkg/BaseLib: Add an additional check within AsciiStriCmp

MdePkg/BaseLib: Add an additional check within AsciiStriCmp

This commit adds an addtional check in AsciiStriCmp. It
explicitly checks the end of the sting pointed by 'SecondString' to make
the code logic easier for reading and to prevent possible mis-reports by
static code checkers.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Hao Wu <Hao.a.wu@intel.com>