[BUG] fix segfault on exit in new appsession code

The new appsession code didn't like it when appsession_hash_destroy()
was called with an empty hash table. Simply add the check.
diff --git a/src/sessionhash.c b/src/sessionhash.c
index dc32820..eb5c213 100644
--- a/src/sessionhash.c
+++ b/src/sessionhash.c
@@ -92,6 +92,9 @@
 	unsigned int i;
 	appsess *item;
 
+	if (!hash->table)
+		return;
+
 	for (i = 0; i < TABLESIZE; i++) {
 		while (!LIST_ISEMPTY(&hash->table[i])) {
 			item = LIST_ELEM(hash->table[i].n, appsess *,