MdeModulePkg/UefiHiiLib: Fix incorrect check for string length
MdeModulePkg/UefiHiiLib: Fix incorrect check for string length

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=681

For string opcode,when checking the valid string length,
it should exclude the Null-terminated character.
And for string in NameValue storage, need to exclude
the varname and also need to convert the Config string
length to Unicode string length.

Cc: Eric Dong <fa9eeec52367040ac98013bc4b2913db170588f8@intel.com>
Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <1e8692d8ed8470b17e8ddd7612a0f029768dcb56@intel.com>
Reviewed-by: Eric Dong <fa9eeec52367040ac98013bc4b2913db170588f8@intel.com>

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