MB-11462 Fix missing state assignment after flow control check

The state variable tracks how much user has read from the buffer.
Return value from check_and_send_buffer_ack() method has to be returned
as the new state for handle_info callback.

Change-Id: Id2f2989b1453c1edbdaa998e13e4f8ed3a4fad4f
Reviewed-on: http://review.couchbase.org/38639
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Nimish Gupta <nimish@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>