fix possible nil

go.mod

@@ -15,7 +15,7 @@ require (
 	github.com/buraksezer/consistent v0.0.0-20191006190839-693edf70fd72
 	github.com/bwmarrin/snowflake v0.3.0
 	github.com/cespare/xxhash v1.1.0
-	github.com/chrislusf/raft v1.0.6
+	github.com/chrislusf/raft v1.0.7
 	github.com/coreos/go-semver v0.3.0 // indirect
 	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/disintegration/imaging v1.6.2