avoid infinite loop in sync when a mzc-generated function in an event generator for a struct, and it returns the struct itself

svn: r3262

src/mzscheme/src/struct.c

@@ -804,6 +804,8 @@ static int evt_struct_is_ready(Scheme_Object *o, Scheme_Schedule_Info *sinfo)
       result = scheme_apply(f, 1, a);
 
       if (scheme_is_evt(result)) {
+	SCHEME_USE_FUEL(1); /* Needed beause an apply of a mzc-generated function
+			       might not check for breaks. */
 	scheme_set_sync_target(sinfo, result, NULL, NULL, 0, 1);
 	return 0;
       }