vm: avoid allocator lookup for singleton class initialize methods

This fixes a SIGABRT of Assertion Failed: RCLASS_ALLOCATOR:!RCLASS_SINGLETON_P(klass)

Author: rwstauner

test/ruby/test_class.rb

@@ -888,6 +888,16 @@ class C; end
     end;
   end
 
+  def test_define_singleton_initialize
+    assert_normal_exit "#{<<~"begin;"}\n#{<<~'end;'}"
+    begin;
+      class C
+        def self.initialize
+        end
+      end
+    end;
+  end
+
   def test_singleton_cc_invalidation
     assert_separately([], "#{<<~"begin;"}\n#{<<~"end;"}")
     begin;

vm_insnhelper.c

@@ -6048,7 +6048,9 @@ vm_define_method(const rb_execution_context_t *ec, VALUE obj, ID id, VALUE iseqv
 
     rb_add_method_iseq(klass, id, (const rb_iseq_t *)iseqval, cref, visi);
     // Set max_iv_count on klasses based on number of ivar sets that are in the initialize method
-    if (id == idInitialize && klass != rb_cObject &&  RB_TYPE_P(klass, T_CLASS) && (rb_get_alloc_func(klass) == rb_class_allocate_instance)) {
+    if (id == idInitialize && klass != rb_cObject && RB_TYPE_P(klass, T_CLASS) &&
+        !RCLASS_SINGLETON_P(klass) &&
+        (rb_get_alloc_func(klass) == rb_class_allocate_instance)) {
         RCLASS_SET_MAX_IV_COUNT(klass, rb_estimate_iv_count(klass, (const rb_iseq_t *)iseqval));
     }