PCBC-191 Don't coredump for invalid argument

Change-Id: I2965068c843965b0ee87d1e81aad1090de2febae
Reviewed-on: http://review.couchbase.org/25432
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>