Network error while DCP streaming will cause Go panic

When a network error occurs on an open connection, all requests which are sitting in the connections opaque map will be sent an ErrNetwork error which is visible here:
https://github.com/couchbase/gocbcore/blob/master/memdpipeline.go#L267

OpenStream creates a PERSISTENT request, which sits in the above mentioned queue for long periods of time, if the above ErrNetwork callback is invoked, the handler for OpenStream will immediately panic due to trying to dereference a nil pointer here:
https://github.com/couchbase/gocbcore/blob/master/agentops_dcp.go#L44