ShellPkg: Fix Shell ASSERT when mv file with cwd is NULL.

ShellPkg: Fix Shell ASSERT when mv file with cwd is NULL.

Shell will ASSERT when doing the following operation:
""
Shell> mv fs2:\file1 fs2:\file2
""
This patch add NULL pointer check to fix this issue.

Fixes: https://github.com/tianocore/edk2/issues/76
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>