Issue #1613 - foreground select tool raises a CRITICAL when committing

Make gimp_free_select_tool_halt() protected, and call it in
gimp_foreground_select_tool_set_trimap(), so that the free-select
subobject of the foreground-select tool is properly shut down
before switching to trimap mode.  In particular, this clears the
free-select tool widget at the right point; failing to do this
leads to CRITICALs later on.

(cherry picked from commit e15733236c)

app/tools/gimpforegroundselecttool.c

@@ -1025,6 +1025,8 @@ gimp_foreground_select_tool_set_trimap (GimpForegroundSelectTool *fg_select)
 
   options = GIMP_FOREGROUND_SELECT_TOOL_GET_OPTIONS (tool);
 
+  gimp_free_select_tool_halt (GIMP_FREE_SELECT_TOOL (fg_select));
+
   gimp_foreground_select_options_get_mask_color (options, &color);
   gimp_display_shell_set_mask (gimp_display_get_shell (tool->display),
                                fg_select->trimap, 0, 0, &color, TRUE);

app/tools/gimpfreeselecttool.c

@@ -233,7 +233,7 @@ gimp_free_select_tool_start (GimpFreeSelectTool *fst,
   gimp_draw_tool_start (GIMP_DRAW_TOOL (tool), display);
 }
 
-static void
+void
 gimp_free_select_tool_halt (GimpFreeSelectTool *fst)
 {
   GimpFreeSelectToolPrivate *private = fst->private;

app/tools/gimpfreeselecttool.h

@@ -61,5 +61,9 @@ GType   gimp_free_select_tool_get_type     (void) G_GNUC_CONST;
 
 gint    gimp_free_select_tool_get_n_points (GimpFreeSelectTool *tool);
 
+/*  protected functions */
+
+void    gimp_free_select_tool_halt         (GimpFreeSelectTool *tool);
+
 
 #endif  /*  __GIMP_FREE_SELECT_TOOL_H__  */