ShellPkg/map: Fix out-of-bound read when "map fsn"

ShellPkg/map: Fix out-of-bound read when "map fsn"

The below code reads additional one CHAR16 when copying
content from Specific to NewSpecific.
NewSpecific = AllocateCopyPool(
                StrSize(Specific) + sizeof(CHAR16), Specific
                );

The patch fixes this issue.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>