ShellPkg/Hexedit: Fix FreePool() ASSERT() when writing disk

ShellPkg/Hexedit: Fix FreePool() ASSERT() when writing disk

The HDiskImageSave() function copies a device path using
DuplicateDevicePath() and passes that device path to
gBS->LocateDevicePath() that changes the value of the
device path pointer.  When FreePool() is called with the
modified device path pointer, the FreePool() service
generates an ASSERT() because the signature for the pool
head can not be found.

The function HDiskImageRead() immediately above
HDiskImageSave() has the correct algorithm that uses an
additional local variable called DupDevicePathForFree to
preserve the pointer to the allocated buffer so it can
be used in the call to FreePool().

Bug: <https://bugzilla.tianocore.org/show_bug.cgi?id=131>

Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>