time_type_impl: value initialize result

parse_time() adds hourse, minutes, etc to a final value 'result'.
However, it is of type std::chrono::nanoseconds which means it is not
zeroed at initialization unless it is explicitly asked to do so.

Fixed debug mode failures in types_tyes and cql_query_test.

Message-Id: <20170125155239.1253-1-pdziepak@scylladb.com>

types.cc

@@ -942,7 +942,7 @@ struct time_type_impl : public simple_type_impl<int64_t> {
                 throw marshal_exception("Nanosecond out of bounds.");
             }
         }
-        std::chrono::nanoseconds result;
+        std::chrono::nanoseconds result{};
         result += std::chrono::hours(hours);
         result += std::chrono::minutes(minutes);
         result += std::chrono::seconds(seconds);