MdeModulePkg CapsuleApp: Check Buffer against NULL before freeing it

If the capsule from command line is not present,
Buffer will be random value when freeing it in DumpCapsule(),
then ASSERT will happen or other memory pool may be freed.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>