Limit max username/password size in SSecurityPlain.

Limit max username/password size in SSecurityPlain.

Setting the limit to 1024 which should be still more than enough.

Unlimited ulen and plen can cause various security problems:
  * Overflow in `is->checkNoWait(ulen + plen)` causing it to contine when there is not enough data and then wait forever.
  * Overflow in `new char[plen + 1]` that would allocate zero sized array which succeeds but returns pointer that should not be written into.
  * Allocation failure in `new char[plen + 1]` from trying to allocate too much and crashing the whole server.

All those issues can be triggered by a client before authentication.