Make sure that the entity to printes is valid.

Especially after savegame load not all slots in the edicts array may be
used. Check if the current entity has a classname string. This is
another part of yquake2/yquake2#430.

src/g_cmds.c

@@ -1368,6 +1368,12 @@ Cmd_ListEntities_f(edict_t *ent)
 		edict_t *cur = &g_edicts[i];
 		qboolean print = false;
 
+		/* Ensure that the entity is valid. */
+		if (!cur->classname)
+		{
+			continue;
+		}
+
 		if (all)
 		{
 			print = true;