address raft nil snapshot during flush

fix https://github.com/chrislusf/seaweedfs/issues/1718

go.mod

@@ -11,7 +11,7 @@ require (
 	github.com/aws/aws-sdk-go v1.33.5
 	github.com/buraksezer/consistent v0.0.0-20191006190839-693edf70fd72
 	github.com/cespare/xxhash v1.1.0
-	github.com/chrislusf/raft v1.0.3
+	github.com/chrislusf/raft v1.0.4
 	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

go.sum

@@ -86,6 +86,8 @@ github.com/cespare/xxhash/v2 v2.1.1 h1:6MnRN8NT7+YBpUIWxHtefFZOKTAPgGjpQSxqLNn0+
 github.com/cespare/xxhash/v2 v2.1.1/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/chrislusf/raft v1.0.3 h1:11YrnzJtVa5z7m9lhY2p8VcPHoUlC1UswyoAo+U1m1k=
 github.com/chrislusf/raft v1.0.3/go.mod h1:Ep5DP+mJSosjfKiix1uU7Lc2Df/SX4oGJEpZlXH5l68=
+github.com/chrislusf/raft v1.0.4 h1:THhbsVik2hxdE0/VXX834f64Wn9RzgVPp+E+XCWZdKM=
+github.com/chrislusf/raft v1.0.4/go.mod h1:Ep5DP+mJSosjfKiix1uU7Lc2Df/SX4oGJEpZlXH5l68=
 github.com/clbanning/x2j v0.0.0-20191024224557-825249438eec/go.mod h1:jMjuTZXRI4dUb/I5gc9Hdhagfvm9+RyrPryS/auMzxE=
 github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
 github.com/cncf/udpa/go v0.0.0-20191209042840-269d4d468f6f/go.mod h1:M8M6+tZqaGXZJjfX53e64911xZQV5JYwmTeXPW+k8Sc=