porting vault fix from consul-template

Fixes issue with vault's API returning pointer types for data field.
Requires a nil check.

Consul-template PR with more info.
hashicorp/consul-template#1447

internal/dependency/vault_common.go

@@ -1,6 +1,7 @@
 package dependency
 
 import (
+	"fmt"
 	"math/rand"
 	"path"
 	"strings"
@@ -258,6 +259,9 @@ func isKVv2(client *api.Client, path string) (string, bool, error) {
 	if err != nil {
 		return "", false, err
 	}
+	if secret == nil {
+		return "", false, fmt.Errorf("secret at path %s does not exist", path)
+	}
 	var mountPath string
 	if mountPathRaw, ok := secret.Data["path"]; ok {
 		mountPath = mountPathRaw.(string)