ShellPkg: Fix bug that fails to change CWD after "map -r".

ShellPkg: Fix bug that fails to change CWD after "map -r".

When "map -r" runs, the mapping list is re-created but
gShellCurMapping still points to the old mapping list which
is already destroyed.
The patch updates the gShellCurMapping to point to the correct
location in the new mapping list.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Huajing Li <huajing.li@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by Jaben Carsey <jaben.carsey@intel.com>