Move Variant#constructor_wrap override to #store_pointer

lib/ffi-glib.rb

@@ -48,5 +48,6 @@ module Lib
     attach_function :g_ptr_array_foreach, [:pointer, Func, :pointer], :pointer
 
     attach_function :g_iconv_open, [:pointer, :pointer], :pointer
+    attach_function :g_variant_ref_sink, [:pointer], :pointer
   end
 end

lib/ffi-glib/variant.rb

@@ -9,8 +9,15 @@ def get_string_with_override
       get_string_without_override.first
     end
 
-    def self.constructor_wrap ptr
-      super.tap(&:ref)
+    # Initializing method used in constructors. For Variant, this needs to sink
+    # the variant's floating reference.
+    #
+    # NOTE: This is very hard to test since it is not possible to get the
+    # variant's ref count directely. However, there is an error when running
+    # the tests on 32-bit systems.
+    def store_pointer ptr
+      super
+      ::GLib::Lib.g_variant_ref_sink ptr
     end
 
     alias_method :get_string_without_override, :get_string