Windows complain about unitialized variable

In the case where inputpos == inputlen it will set pwlen to
0 bytes, but the windows runtime that checks for unitialized use
does not check if it is ever actually being used when it
calls the function cbsasl_secure_compare, just that it pass an
uninitialized variable to the function.

Change-Id: I8fed01c1d143d850cefe265988da8f093c4ba49c
Reviewed-on: http://review.couchbase.org/32356
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>