Make SingleResult return an error on nil registry

If the registry for SingleResult is nil, return bson.ErrNilRegistry.

GODRIVER-696

Change-Id: Iea3de38c9cc2c4a348b6614f760685aadc842157

Author: skriptble

mongo/command_monitoring_test.go

@@ -55,6 +55,7 @@ func createMonitoredClient(t *testing.T, monitor *event.CommandMonitor) *Client
 		connString:     testutil.ConnString(t),
 		readPreference: readpref.Primary(),
 		clock:          &session.ClusterClock{},
+		registry:       bson.DefaultRegistry,
 	}
 }
 

mongo/sessions_test.go

@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"strings"
 
+	"github.com/mongodb/mongo-go-driver/bson"
 	"github.com/mongodb/mongo-go-driver/event"
 	"github.com/mongodb/mongo-go-driver/internal/testutil"
 	"github.com/mongodb/mongo-go-driver/internal/testutil/helpers"
@@ -229,6 +230,7 @@ func createSessionsMonitoredClient(t *testing.T, monitor *event.CommandMonitor)
 		readPreference: readpref.Primary(),
 		readConcern:    readconcern.Local(),
 		clock:          clock,
+		registry:       bson.DefaultRegistry,
 	}
 
 	subscription, err := c.topology.Subscribe()

mongo/single_result.go

@@ -33,6 +33,9 @@ type SingleResult struct {
 // will be returned. If there were no returned documents, ErrNoDocuments is
 // returned.
 func (sr *SingleResult) Decode(v interface{}) error {
+	if sr.reg == nil {
+		return bson.ErrNilRegistry
+	}
 	switch {
 	case sr.err != nil:
 		return sr.err