CCBC-188 Fix segfault when rebalancing

When a (!connected) server is reconnected, the tasks in its "pending"
buffer will be moved into "output" buffer. If it's connection is
broken again immediately, relocate_packets() will go to wrong path.

Change-Id: I6d4cccd20e16f80176132c93fa7e2890950a2630
Reviewed-on: http://review.couchbase.org/25852
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>