terminal: sysview: don't return uninitialized error codes

In case 'scan_evdev' and 'scan_drm' are both false, we never set 'r' to
anyhting, thus return an uninitialized error code. Fix this by always
returning 0 as we catch negative codes earlier, anyway. Thanks to Thomas
H.P. Anderson for the report.

src/libsystemd-terminal/sysview.c

@@ -821,7 +821,7 @@ static int context_ud_prepare_monitor(sysview_context *c, struct udev_monitor *m
                         return r;
         }
 
-        return r;
+        return 0;
 }
 
 static int context_ud_prepare_scan(sysview_context *c, struct udev_enumerate *e) {