fix a memleak in tiff pyr to memory

we were not freeing layers other than the top layer

ChangeLog

@@ -3,6 +3,7 @@
 - better magickload error messages
 - more consistent behaviour for page-height metadata
 - fix for composite with many small images and some combinations of blend modes
+- fix memleak in tiff pyr save to memory [scossu]
 
 21/9/18 started 8.8.0
 - much faster smartcrop [lovell]

libvips/foreign/tiff.c

@@ -418,6 +418,9 @@ vips__tiff_openout_buffer( VipsImage *image,
 	printf( "vips__tiff_openout_buffer:\n" );
 #endif /*DEBUG*/
 
+	g_assert( out_data );
+	g_assert( out_length );
+
 	buffer = VIPS_NEW( image, VipsTiffOpenoutBuffer );
 	vips_dbuf_init( &buffer->dbuf );
 	buffer->out_data = out_data;

libvips/foreign/vips2tiff.c

@@ -835,6 +835,7 @@ layer_free( Layer *layer )
 	VIPS_UNREF( layer->strip );
 	VIPS_UNREF( layer->copy );
 	VIPS_UNREF( layer->image );
+	VIPS_FREE( layer->buf );
 	VIPS_FREEF( TIFFClose, layer->tif );
 }