pstree: check if thread core is allocated before freeing

It looks like, it's possible that cores for some threads
were allocated, but not for all - allocation failed in
pstree_alloc_cores(). And after that we will dereference
NULL pointer as pstree_free_cores() doesn't check pointer:

pstree.c:28:6: warning: Access to field 'tc' results in a dereference of a null pointer (loaded from variable 'core')
        if (core->tc && core->tc->timers)
            ^~~~~~~~
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/pstree.c

@@ -163,7 +163,8 @@ void pstree_free_cores(struct pstree_item *item)
 
 	if (item->core) {
 		for (i = 1; i < item->nr_threads; i++)
-			core_entry_free(item->core[i]);
+			if (item->core[i])
+				core_entry_free(item->core[i]);
 		xfree(item->core);
 		item->core = NULL;
 	}