MdePkg/BaseLib: Add an additional check within (Ascii)StrnCmp

MdePkg/BaseLib: Add an additional check within (Ascii)StrnCmp

This commit adds an addtional check in AsciiStrnCmp and StrnCmp. 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 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>