fix possible memleak

fix tiny leak if output options failed

TODO

@@ -1,13 +1,3 @@
-- try:
-
-	$ vips system "vips add %s %s %s" --in "k2.jpg k4.jpg" \
-		--out-format "%s.tif" --out x.v --log
-	memory: high-water mark 9.37 MB
-	""
-	memory: high-water mark 27.89 MB
-	$
-
-  but x.v does not appear
 
 - can we use postbuild elsewhere? look at use of "preclose" / "written", etc. 
 

libvips/iofuncs/operation.c

@@ -793,10 +793,14 @@ vips_call_option_output( VipsObject *object,
 		result = vips_object_get_argument_to_string( object, 
 			g_param_spec_get_name( pspec ), output->value );
 
+	return( result ); 
+}
+
+static void
+vips_call_option_output_free( VipsObject *object, VipsCallOptionOutput *output )
+{
 	VIPS_FREE( output->value ); 
 	g_free( output );
-
-	return( result ); 
 }
 
 static gboolean
@@ -866,6 +870,9 @@ vips_call_options_set( const gchar *option_name, const gchar *value,
 		g_signal_connect( operation, "postbuild",
 			G_CALLBACK( vips_call_option_output ),
 			output );
+		g_signal_connect( operation, "close",
+			G_CALLBACK( vips_call_option_output_free ),
+			output );
 	}
 
 	return( TRUE );