fix `defined?(@ivar)` with Ractors

`defined?(@ivar)` on the non main Ractor has two issues:

1. raising an exception

```ruby
class C
  @iv1 = []
  def self.defined_iv1 = defined?(@iv1)
end

Ractor.new{
  p C.defined_iv1
  #=> can not get unshareable values from instance variables of classes/modules from non-main Ractors (Ractor::IsolationError)
}.take
```

-> Do not raise an exception but return `"instance-variable"` because
it is defined.

2. returning `"instance-variable"` if there is not defined.

```
class C
  # @iv2 is not defined
  def self.defined_iv2 = defined?(@iv2)
end

Ractor.new{
  p C.defined_iv2 #=> "instance-variable"
}.take
```

-> returns `nil`

bootstraptest/test_ractor.rb

@@ -1086,6 +1086,20 @@ assert_equal '333', %q{
   a + b + c + d + e + f
 }
 
+assert_equal '["instance-variable", "instance-variable", nil]', %q{
+  class C
+    @iv1 = ""
+    @iv2 = 42
+    def self.iv1 = defined?(@iv1) # "instance-variable"
+    def self.iv2 = defined?(@iv2) # "instance-variable"
+    def self.iv3 = defined?(@iv3) # nil
+  end
+
+  Ractor.new{
+    [C.iv1, C.iv2, C.iv3]
+  }.take
+}
+
 # moved objects have their shape properly set to original object's shape
 assert_equal '1234', %q{
 class Obj

vm_insnhelper.c

@@ -1252,7 +1252,13 @@ vm_getivar(VALUE obj, ID id, const rb_iseq_t *iseq, IVC ic, const struct rb_call
                 // and modules. So we can skip locking.
                 // Second, other ractors need to check the shareability of the
                 // values returned from the class ivars.
-                goto general_path;
+
+                if (default_value == Qundef) { // defined?
+                    return rb_ivar_defined(obj, id) ? Qtrue : Qundef;
+                }
+                else {
+                    goto general_path;
+                }
             }
 
             ivar_list = RCLASS_IVPTR(obj);