Add missing error check on BN_CTX_new()

If this fails, then the big numbers will be allocated outside of the
context, leading to leaks.

Closes phpGH-21061.

Author: ndossche

ext/openssl/openssl.c

@@ -4423,6 +4423,9 @@ static bool php_openssl_pkey_init_legacy_ec(EC_KEY *eckey, zval *data, bool *is_
 	EC_POINT *point_q = NULL;
 	EC_GROUP *group = NULL;
 	BN_CTX *bctx = BN_CTX_new();
+	if (!bctx) {
+		goto clean_exit;
+	}
 
 	*is_private = false;