fix warning for unused vips7 gvalue arg

this fixes this warning:

$ vips im_copy_set_meta
GLib-GObject-CRITICAL **: g_value_unset: assertion `G_IS_VALUE (value)'
failed

ChangeLog

@@ -1,5 +1,6 @@
 6/4/12 started 7.28.3
 - vips_divide() failed for int arguments
+- fix warning for unused vips7 gvalue argument
 
 13/3/12 started 7.28.2
 - xres/yres tiffsave args were broken

libvips/deprecated/dispatch_types.c

@@ -817,7 +817,11 @@ gvalue_free( im_object obj )
 {
 	GValue *value = obj;
 
-	g_value_unset( value );
+	/* If it's just zeros (built but not used) we'll get an error if we
+	 * unset().
+	 */
+	if( G_IS_VALUE( value ) )
+		g_value_unset( value );
 
 	return( 0 );
 }