tryObjectEncoding(): don't call stringl2() for too big strings.
tryObjectEncoding(): don't call stringl2() for too big strings.

We are sure that a string that is longer than 21 chars cannot be
represented by a 64 bit signed integer, as -(2^64) is 21 chars:

strlen(-18446744073709551616) => 21

DeNA/redis,DeNA/redis,DeNA/redis