avoid possible nil entry.pb

I0410 15:17:53 72441 volume_layout.go:343] Volume 390 becomes unwritable
I0410 15:17:53 72441 volume_layout.go:343] Volume 391 becomes unwritable
I0410 15:17:53 72441 volume_layout.go:343] Volume 394 becomes unwritable
I0410 15:17:53 72441 volume_layout.go:343] Volume 392 becomes unwritable
I0410 15:17:58 72441 volume_layout.go:343] Volume 393 becomes unwritable
I0410 15:17:58 72441 volume_layout.go:343] Volume 395 becomes unwritable
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

Author: chrisluuber

server.go

@@ -1273,8 +1273,11 @@ func (s *server) TakeSnapshot() error {
 	// We do not want to send the whole snapshot to the slightly slow machines
 	if lastIndex-s.log.startIndex > NumberOfLogEntriesAfterSnapshot {
 		compactIndex := lastIndex - NumberOfLogEntriesAfterSnapshot
-		compactTerm := s.log.getEntry(compactIndex).Term()
-		s.log.compact(compactIndex, compactTerm)
+		entry := s.log.getEntry(compactIndex)
+		if entry.pb != nil {
+			compactTerm := entry.Term()
+			s.log.compact(compactIndex, compactTerm)
+		}
 	}
 
 	return nil