correct sloppy NULL checks, in interface code:

- uiPupMenuBegin was checking for NULL title but would still crash if it were NULL.
- now tag the function as 'nonnull' and remove NULL checks. make sure NULL isnt passed to it.

source/blender/editors/include/UI_interface.h

@@ -327,7 +327,11 @@ typedef void (*uiMenuHandleFunc)(struct bContext *C, void *arg, int event);
 
 typedef struct uiPopupMenu uiPopupMenu;
 
-struct uiPopupMenu *uiPupMenuBegin(struct bContext *C, const char *title, int icon);
+struct uiPopupMenu *uiPupMenuBegin(struct bContext *C, const char *title, int icon)
+#ifdef __GNUC__
+__attribute__((nonnull))
+#endif
+;
 void uiPupMenuEnd(struct bContext *C, struct uiPopupMenu *head);
 struct uiLayout *uiPupMenuLayout(uiPopupMenu *head);