better error detection in vips_system()

ChangeLog

@@ -4,6 +4,7 @@
 - vips_scale() now does round to nearest to avoid rounding errors
 - improve im_openout() compat macro
 - more vips7 compatibility fixes, thanks steve
+- more robust vips_system()
 
 30/6/14 started 7.40.3
 - fix interlaced thumbnails in vipsthumbnail, thanks lovell

libvips/iofuncs/system.c

@@ -177,7 +177,8 @@ vips_system_build( VipsObject *object )
 			memmove( p, p + 1, strlen( p ) );
 
 	if( !g_spawn_command_line_sync( cmd, 
-		&std_output, &std_error, &result, &error ) ) {
+		&std_output, &std_error, &result, &error ) ||
+		result ) {
 		if( error ) {
 			vips_error( class->nickname, "%s", error->message );
 			g_error_free( error );
@@ -202,8 +203,6 @@ vips_system_build( VipsObject *object )
 		return( -1 ); 
 	}
 
-	g_assert( !result ); 
-
 	if( std_error ) {
 		vips__chomp( std_error ); 
 		if( strcmp( std_error, "" ) != 0 )