MdeModulePkg CapsuleApp: ASSERT to ensure 'CapsuleIndex' is not NULL

MdeModulePkg CapsuleApp: ASSERT to ensure 'CapsuleIndex' is not NULL

Function GetVariable2() ensures its third (output) parameter will not be
NULL when the return status is EFI_SUCCESS.

This commit adds ASSERT as warnings for the case that will not happen.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>