Revise TROVE-2020-002 fix to work on older OpenSSL versions.

Although OpenSSL before 1.1.1 is no longer supported, it's possible
that somebody is still using it with 0.3.5, so we probably shouldn't
break it with this fix.

src/lib/crypt_ops/crypto_rsa_openssl.c

@@ -584,7 +584,11 @@ crypto_pk_asn1_decode_private(const char *str, size_t len, int max_bits)
     crypto_openssl_log_errors(LOG_WARN,"decoding private key");
     return NULL;
   }
+#ifdef OPENSSL_1_1_API
   if (max_bits >= 0 && RSA_bits(rsa) > max_bits) {
+#else
+  if (max_bits >= 0 && rsa->n && BN_num_bits(rsa->n) > max_bits) {
+#endif
     log_info(LD_CRYPTO, "Private key longer than expected.");
     return NULL;
   }