[st-scroll-view.c] avoid double disconnect

The console has recently started displaying the following message just after Cinnamon has started:

````
Window manager warning: Log level 16: /build/buildd/glib2.0-2.32.3/./gobject/gsignal.c:2572: instance `0x7829000' has no handler with id `20365'
````

The enclosed patch seems to silence the warning, with the disclaimer that I am not at all familiar with the GObject object model.