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 <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>