MB-8466: Memcached crashes is sasl pwlen < 0

The current code has an assertion to check that the pwlen is
greater than 0. This means memcached crashes if we see a packet
with that value. Instead we should just return EInvalid to the
client and allow memcached to keep running.

Change-Id: I76316a2cef572e71d9e690966d996927032ade19
Reviewed-on: http://review.couchbase.org/26975
Tested-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>