ShellPkg/hexedit: Fix a read-after-free bug

HDiskImageSetDiskNameOffsetSize() and HFileImageSetFileName()
may be called using the current disk name or file name.
When this happens, today's implementation firstly frees the memory
and then accesses the just-freed memory.
The patch fixes this issue by doing nothing when the disk or file
name is the current one.

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