SignedCapsulePkg: Address NULL pointer dereference case.

SignedCapsulePkg: Address NULL pointer dereference case.

Original code GetFmpImageDescriptors for OriginalFmpImageInfoBuf
pointer, if failed, return a NULL pointer. The OriginalFmpImageInfoBuf
should not be NULL and the NULL pointer dereference case
should be false positive.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Vin Xue <vinxue@outlook.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>