Fix off-by-one with RubyVM::Shape.exhaust_shapes

Previously, the method left one shape available (MAX_SHAPE_ID) when
called without arguments.

shape.c

@@ -1062,7 +1062,7 @@ rb_shape_exhaust(int argc, VALUE *argv, VALUE self)
 {
     rb_check_arity(argc, 0, 1);
     int offset = argc == 1 ? NUM2INT(argv[0]) : 0;
-    GET_SHAPE_TREE()->next_shape_id = MAX_SHAPE_ID - offset;
+    GET_SHAPE_TREE()->next_shape_id = MAX_SHAPE_ID - offset + 1;
     return Qnil;
 }