ipn: fix crash generating machine key on new installs

Regression from d6ad41dcea (for #732).

Probably also means eab6e9ea4e was unnecessary, but it's fine.

Fixes #887

ipn/local.go

@@ -708,7 +708,9 @@ func (b *LocalBackend) initMachineKeyLocked() (err error) {
 			if err != nil {
 				return
 			}
-			b.prefs.Persist.LegacyFrontendPrivateMachineKey = b.machinePrivKey
+			if b.prefs != nil && b.prefs.Persist != nil {
+				b.prefs.Persist.LegacyFrontendPrivateMachineKey = b.machinePrivKey
+			}
 		}()
 	}