BUG/MEDIUM: stick-table: fix regression caused by recent fix for out-of-memory

Commit ef8f4fe ("BUG/MINOR: stick-table: handle out-of-memory condition
gracefully") unfortunately got trapped by a pointer operation. Replacing

    ts = poll_alloc() + size;

with :

    ts = poll_alloc();
    ts += size;

Doesn't give the same result because pool_alloc() is void while ts is a
struct stksess*. So now we don't access the same places, which is visible
in certain stick-table scenarios causing a crash.

This must be backported to 1.6 and 1.5.

src/stick_table.c

@@ -173,7 +173,7 @@ struct stksess *stksess_new(struct stktable *t, struct stktable_key *key)
 	ts = pool_alloc2(t->pool);
 	if (ts) {
 		t->current++;
-		ts += t->data_size;
+		ts = (void *)ts + t->data_size;
 		stksess_init(t, ts);
 		if (key)
 			stksess_setkey(t, ts, key);