ShellPkg: Fix the ASSERT issue in Shell 'for' loop

ShellPkg: Fix the ASSERT issue in Shell 'for' loop

The Length parameter of 'GetNextParameter' is the buffer size in bytes.
While StrnCpys requires user to pass the max number of dest unicode char,
we should convert size in bytes to the number of char.

Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
[lersek@redhat.com: updated commit message as requested by Jaben]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18059 6f19259b-4bc3-4df7-8a09-765794883524