Fixes incorrect grabbing behaviour causing subsequent rejection of input

mouse_location can be set to NULL in gtk_range_update_mouse_location(). This
causes a match with stepper_?_gadget in gtk_range_multipress_gesture_pressed(),
as both are NULL. This leads to a grab being added internally and as well, a
critical error message, and a subsequent rejection of touch inputs. Returning
early when mouse_location is NULL prevents this.

Closes #4947

gtk/gtkrange.c

@@ -2711,6 +2711,11 @@ gtk_range_multipress_gesture_pressed (GtkGestureMultiPress *gesture,
   priv->mouse_y = y;
 
   gtk_range_update_mouse_location (range);
+  if (!priv->mouse_location)
+    {
+      g_warning(G_STRLOC "value of mouse_location is 'NULL'");
+      return;
+    }
   gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
 
   g_object_get (gtk_widget_get_settings (widget),