Make recalculate_sizes() handle an empty window with no active

pane. This can happen when a window is in two sessions - it isn't
destroyed immediately when the pane goes away but is left until the last
session is destroyed. Fixes problems with grouped sessions reported by
Daniel Ralston.

resize.c

@@ -92,7 +92,7 @@ recalculate_sizes(void)
 
 	for (i = 0; i < ARRAY_LENGTH(&windows); i++) {
 		w = ARRAY_ITEM(&windows, i);
-		if (w == NULL)
+		if (w == NULL || w->active == NULL)
 			continue;
 		flag = options_get_number(&w->options, "aggressive-resize");