MdePkg: ensure SafeString length functions don't access beyond MaxSize

The StrnLenS and AsciiStrnLenS functions, when presented with a string
with no terminating NULL in the first MaxSize characters will check
the character at String[MaxSize] before checking if Length < MaxSize.
(They return the correct value, but have accessed beyond the stated
limit in the process.)

Flip the order of the tests to prevent this behaviour.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17936 6f19259b-4bc3-4df7-8a09-765794883524

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