MdeModulePkg/CapsuleApp: Fix logic bug in CleanGatherList()

MdeModulePkg/CapsuleApp: Fix logic bug in CleanGatherList()

https://bugzilla.tianocore.org/show_bug.cgi?id=905

Fix pointer math when more than one capsule is passed
to the CapsuleApp.  Use the ContinuationPointer from
the last array entry instead of the first array entry.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>