raft: fix possible nil
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1962c8a]
goroutine 68239935 [running]:
github.com/chrislusf/raft.(*LogEntry).Term(...)
/home/travis/gopath/pkg/mod/github.com/chrislusf/raft@v1.0.4/log_entry.go:59
github.com/chrislusf/raft.(*server).TakeSnapshot(0xc00014f320, 0xc028b5a600, 0xc03995bc00)
/home/travis/gopath/pkg/mod/github.com/chrislusf/raft@v1.0.4/server.go:1276 +0x50a
github.com/chrislusf/raft.(*server).maybeTakeSnapshot.func1(0xc00014f320)
/home/travis/gopath/pkg/mod/github.com/chrislusf/raft@v1.0.4/server.go:1221 +0x5b
created by github.com/chrislusf/raft.(*server).maybeTakeSnapshot
/home/travis/gopath/pkg/mod/github.com/chrislusf/raft@v1.0.4/server.go:1219 +0x98