CryptoPkg/BaseCryptLib: Add NULL pointer checks in DH and P7Verify

CryptoPkg/BaseCryptLib: Add NULL pointer checks in DH and P7Verify

Add more NULL pointer checks before using them in DhGenerateKey and
Pkcs7GetCertificatesList functions to eliminate possible dereferenced
pointer issue.

Cc: Ting Ye <ting.ye@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qin Long <qin.long@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ting Ye <ting.ye@intel.com>