MB-18872: Add guard for access to mech member
MB-18872: Add guard for access to mech member

Stack backtrace of crashed thread:
     [ ... cut ... ]
     /lib64/libpthread.so.0() [0x7f95ad784790]
     libcbsasl.so.1.1.1(cbsasl_server_step+0x15) [0x7f95ae1cb525]
     [ ... cut ... ]

I don't have access to the core files so I can't verify that
this is the root cause for the bug, but the code tries to
dereference a unique_ptr which might not be initialized yet
(but that would be an illegal use of the SASL API. That
we're using sasl_step on a sasl_t which we didn't do
a successful sasl_start).

Change-Id: I4c97f59daaef75c9aaa8922deae1614dfba076e3
Reviewed-on: http://review.couchbase.org/62095
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Will Gardner <cbd6f5d9092d9ef7d5c0510d53bba134b22c02f2@couchbase.com>

couchbase/memcached,couchbase/memcached,owendCB/memcached,daverigby/kv_engine,daverigby/memcached,couchbase/memcached,daverigby/kv_engine,daverigby/memcached,owendCB/memcached,owendCB/memcached,daverigby/memcached,daverigby/kv_engine,daverigby/kv_engine,couchbase/memcached,owendCB/memcached,daverigby/memcached