lnd: prevent panic on nil neutrino cleanup

A cleanup closure is not included when an error is returned, causing the
defer to execute and triggering the following panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x105da38]

goroutine 1 [running]:
github.com/lightningnetwork/lnd.Main(0x2083e40, 0xc0004f6db0)
	/home/user/lnd/lnd.go:208 +0x2bfa
main.main()
	/home/user/lnd/cmd/lnd/main.go:14 +0x26

lnd.go

@@ -203,10 +203,10 @@ func Main() error {
 		neutrinoBackend, neutrinoCleanUp, err := initNeutrinoBackend(
 			mainChain.ChainDir,
 		)
-		defer neutrinoCleanUp()
 		if err != nil {
 			return err
 		}
+		defer neutrinoCleanUp()
 		neutrinoCS = neutrinoBackend
 	}