SIGHUP is only useful when config was loaded from a file (#1030)

Have (*config.C).CatchHUP() return early when there is no file
path available from which to reload.
This will allow wrapping service to manage their own signal
trapping (which is particularly important if they've used
config from a string.

config/config.go

@@ -121,6 +121,10 @@ func (c *C) HasChanged(k string) bool {
 // CatchHUP will listen for the HUP signal in a go routine and reload all configs found in the
 // original path provided to Load. The old settings are shallow copied for change detection after the reload.
 func (c *C) CatchHUP(ctx context.Context) {
+	if c.path == "" {
+		return
+	}
+
 	ch := make(chan os.Signal, 1)
 	signal.Notify(ch, syscall.SIGHUP)