Fix return type error of GetEVPCipher() (#19)

Return a pointer rather than Status for GetEVPCipher() when SM4 not supported.

Author: acelyc111

encryption/encryption.cc

@@ -107,9 +107,7 @@ const EVP_CIPHER* GetEVPCipher(EncryptionMethod method) {
       //      return EVP_aes_256_ecb();
     case EncryptionMethod::kSM4_CTR:
 #if OPENSSL_VERSION_NUMBER < 0x1010100fL || defined(OPENSSL_NO_SM4)
-      return Status::InvalidArgument(
-          "Unsupport SM4 encryption method under OpenSSL version: " +
-          std::string(OPENSSL_VERSION_TEXT));
+      return nullptr;
 #else
       // Openssl support SM4 after 1.1.1 release version.
       return EVP_sm4_ctr();