upr client: Fix a race condition in print_log callback

Stream end can happen independently at any time. Hence
we should not assume that stream information for a request
will be available when print log is requested.

Change-Id: I2bca4cf9b70377e74a6002155604eb46f8b060c9
Reviewed-on: http://review.couchbase.org/39091
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>