vips_image_write() could crash if called twice

two writes in a row could trigger a crash in some circumstances, thanks
@dirceupj

see https://github.com/jcupitt/ruby-vips/issues/156

ChangeLog

@@ -1,5 +1,7 @@
 12/2/18 started 8.6.3
 - use pkg-config to find libjpeg, if we can
+- better clean of output image in vips_image_write() fixes a crash 
+  writing twice to memory
 
 5/1/18 started 8.6.2
 - vips_sink_screen() keeps a ref to the input image ... stops a rare race

libvips/iofuncs/image.c

@@ -2539,7 +2539,7 @@ vips_image_write( VipsImage *image, VipsImage *out )
 		vips_object_local( out, image );
 	}
 	else {
-		vips__reorder_clear( image );
+		vips__reorder_clear( out );
 		vips__link_break_all( out );
 	}