Fix panic when closing conn during cancellable query

fixes #29

internal/ctxwatch/context_watcher_test.go

@@ -59,6 +59,17 @@ func TestContextWatcherMultipleWatchPanics(t *testing.T) {
 	require.Panics(t, func() { cw.Watch(ctx2) }, "Expected panic when Watch called multiple times")
 }
 
+func TestContextWatcherUnwatchIsAlwaysSafe(t *testing.T) {
+	cw := ctxwatch.NewContextWatcher(func() {}, func() {})
+	cw.Unwatch() // unwatch when not / never watching
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	cw.Watch(ctx)
+	cw.Unwatch()
+	cw.Unwatch() // double unwatch
+}
+
 func TestContextWatcherStress(t *testing.T) {
 	var cancelFuncCalls int64
 	var cleanupFuncCalls int64