MdeModulePkg/CapsuleApp: Fix potential NULL pointer dereference issue

MdeModulePkg/CapsuleApp: Fix potential NULL pointer dereference issue

To avoid potential NULL pointer dereference issue. Initialize them at
the beginning of the function. This patch is a supplement which was missed
at e98212cb5d59fff8f385d9179ad7f1a3ce9cf215 commit.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>