ShellPkg/TftpDynamicCommand: Fix the potentially uninitialized local variable used.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1217

Local variable 'Mtftp4Token' might be uninitialized when error happen. This patch is to
resolve the issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Carsey Jaben <jaben.carsey@intel.com>
Cc: Zeng Star <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>