ShellPkg: BcfgDisplayDump(): hoist NULL-init of DevPath[String]

It will help with error handling if we move these initializations near the
top of the loop body.

This patch is not supposed to change behavior.

Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19708 6f19259b-4bc3-4df7-8a09-765794883524