fixed backend tests

backends/postgres/src/postgres_connection.cpp

@@ -20,7 +20,7 @@ postgres_connection::postgres_connection(const sql::connection_info &info)
 }
 
 utils::result<void, utils::error> postgres_connection::open() {
-  if (is_open()) {
+  if (conn_ != nullptr) {
     return utils::ok<void>();
   }
 
@@ -221,7 +221,11 @@ utils::result<bool, utils::error> postgres_connection::exists(const std::string
     return utils::failure(make_error(sql::error_code::TABLE_EXISTS_FAILED, res, conn_, "Failed check if table exists", stmt));
   }
 
-  return utils::ok(utils::to<size_t>(PQcmdTuples(res)) == 1);
+  const auto result = utils::to<size_t>(PQcmdTuples(res));
+  if (!result) {
+    return utils::failure(make_error(sql::error_code::FAILURE, res, conn_, "Failed to convert result value", stmt));
+  }
+  return utils::ok(*result == 1);
 }
 
 std::string postgres_connection::to_escaped_string(const utils::blob& value) const