masks: address maybe uninitialized compiler warning

Although it is actually a false-positive warning by GCC12 in -O3 mode, which is not observed in any more modern toolchain on any optimization level.

Author: da-phil

src/develop/masks/masks.c

@@ -1343,7 +1343,7 @@ gboolean dt_masks_events_mouse_moved(dt_iop_module_t *module,
   // this must be serialized against that read (see history_mutex there).
   _events_lock(darktable.develop);
 
-  dt_masks_form_gui_t before;
+  dt_masks_form_gui_t before = { 0 };
   if(gui) before = *gui;
 
   int rep = 0;