MdeModulePkg/UefiBootManagerLib: Initialize Handle before using it

MdeModulePkg/UefiBootManagerLib: Initialize Handle before using it

BmExpandMediaDevicePath contains a bug that it uses the
uninitialized Handle.

Since the function is called when the Handle supports BlockIo
or SimpleFileSystem, when there is no SimpleFileSystem installed
on the Handle, BlockIo is *guaranteed* to be installed on the Handle.
The fix initializes the Handle by locating the BlockIo protocol
from the device path.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Sunny Wang <sunnywang@hpe.com>