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>

CryptoPkg/Library/BaseCryptLib/Pk/CryptDh.c

@@ -232,7 +232,9 @@ DhGenerateKey (
       return FALSE;
     }
 
-    BN_bn2bin (DhPubKey, PublicKey);
+    if (PublicKey != NULL) {
+      BN_bn2bin (DhPubKey, PublicKey);
+    }
     *PublicKeySize = Size;
   }