rewrite 351ebd7 and 954a8c7 because they cause crash when refer to cl…

…ass variable within instance_{eval, exec}'s block.

Test Script:
{{{
require 'test/unit/assertions.rb'
include Test::Unit::Assertions

@@A = 4
assert_equal(4, 1.instance_eval { @@A } )
assert_equal(4, 1.instance_exec { @@A } )

assert_raise(TypeError){ 1.instance_eval { def foo; end } }
assert_raise(TypeError){ 1.instance_eval { alias :foo :to_s} }

assert_raise(TypeError){ 1.instance_exec { def foo; end } }
assert_raise(TypeError){ 1.instance_exec { alias :foo :to_s} }

puts :ok
}}}

vm.cpp

@@ -1542,9 +1542,9 @@ rb_vm_alias2(VALUE outer, VALUE name, VALUE def, unsigned char dynamic_class)
 	if (k != NULL) {
 	    outer = (VALUE)k;
 	}
-    }
-    if (NIL_P(outer)) {
-	rb_raise(rb_eTypeError, "no class to make alias");
+	else if (RCLASS_SUPER(outer) == 0) {
+	    rb_raise(rb_eTypeError, "no class to make alias");
+	}
     }
 
     // Given arguments should always be symbols (compiled as such).
@@ -2064,9 +2064,9 @@ prepare_method(Class klass, bool dynamic_class, SEL sel, void *data,
 		klass = *(Class *)klass;
 	    }
 	}
-    }
-    if (NIL_P(klass)) {
-	rb_raise(rb_eTypeError, "no class/module to add method");
+	else if (RCLASS_SUPER(klass) == 0) {
+	    rb_raise(rb_eTypeError, "no class/module to add method");
+	}
     }
 
     const long v = RCLASS_VERSION(klass);

vm_eval.c

@@ -513,7 +513,7 @@ rb_obj_instance_eval_imp(VALUE self, SEL sel, VALUE top, int argc, VALUE *argv)
     VALUE klass;
 
     if (SPECIAL_CONST_P(self) || CLASS_OF(self) == rb_cSymbol) {
-	klass = Qnil;
+	klass = 0;
     }
     else {
 	klass = rb_singleton_class(self);
@@ -561,7 +561,7 @@ rb_obj_instance_exec(VALUE self, SEL sel, int argc, VALUE *argv)
     VALUE klass;
 
     if (SPECIAL_CONST_P(self)) {
-	klass = Qnil;
+	klass = 0;
     }
     else {
 	klass = rb_singleton_class(self);