MdeModulePkg/DxeCapsuleLibFmp: Add missing NULL pointer check.

MdeModulePkg/DxeCapsuleLibFmp: Add missing NULL pointer check.

Add missing NULL pointer check for CapsuleNameBufStart.
Also add comments to notice that capsule name integrity check assumes
the capsule has been validated by IsValidCapsuleHeader().

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Chao B Zhang <chao.b.zhang@intel.com>
Signed-off-by: Wei6 Xu <wei6.xu@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Chao B Zhang <chao.b.zhang@intel.com>