Fix build error (nil pointer dereference)

[enobufs]

Your environment.

• Version: v1.3.0
• Browser: n/a

What did you do?

CI reported build error (log shown below)

What did you expect?

Build and tests to pass

What happened?

=== RUN   TestServer/simple
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x68771e]
goroutine 32 [running]:
time.(*Timer).Stop(...)
	/home/travis/.gimme/versions/go1.12.7.linux.amd64/src/time/sleep.go:74
github.com/pion/turn/internal/client.(*Transaction).StopRtxTimer(0xc0001dc070)
	/home/travis/gopath/src/github.com/pion/turn/internal/client/transaction.go:75 +0xbe
github.com/pion/turn.(*Client).handleSTUNMessage(0xc0001b0240, 0xc0001fc000, 0x28, 0xffff, 0x998060, 0xc00020e000, 0xc00020e000, 0x0)
	/home/travis/gopath/src/github.com/pion/turn/client.go:474 +0x99c
github.com/pion/turn.(*Client).HandleInbound(0xc0001b0240, 0xc0001fc000, 0x28, 0xffff, 0x998060, 0xc00020e000, 0xc00020e000, 0x0, 0x0)
	/home/travis/gopath/src/github.com/pion/turn/client.go:400 +0x63c
github.com/pion/turn.(*Client).Listen.func1(0xc0001b0240)
	/home/travis/gopath/src/github.com/pion/turn/client.go:173 +0x18d
created by github.com/pion/turn.(*Client).Listen
	/home/travis/gopath/src/github.com/pion/turn/client.go:164 +0x1e1
FAIL	github.com/pion/turn	1.390s

This never happens on my PC, but I found a typo in the code. The nil check was done against a wrong variable:

// StopRtxTimer stop the transaction timer
func (t *Transaction) StopRtxTimer() {
	t.mutex.Lock()
	defer t.mutex.Unlock()

	if t != nil { // <----------------- :(  this should be t.time
		t.timer.Stop()
	}
}