ShellPkg: Fix bug introduced by r17730.

ShellPkg: Fix bug introduced by r17730.

CurrentFilePattern is only part of FilePattern and will be less than or equal to FilePattern. If we use StrCpyS to replace StrnCpy, it will cause assert when FilePattern is longer.
The bug can be replayed when we cd to one directory and run ls command.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17821 6f19259b-4bc3-4df7-8a09-765794883524