ShellPkg: Refine the code logic of StrnCatGrow.

ShellPkg: Refine the code logic of StrnCatGrow.

In order to indicate the catena length use StrnCatS instead of StrCatS to generate Destination string, and update the DestMax of Destination string.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hp.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17795 6f19259b-4bc3-4df7-8a09-765794883524