HTTP parser: fixed memory overflow in the collisions test.

The level hash uses the NULL value as the indicator of a free entry in a bucket.
So, inserting a NULL value breaks the hash and can lead to a bucket overflow.

In case of the collision counter, the value wasn't initialized, since it's not
needed for the purpose of checking collisions.  As a result, it might contain
any garbage from the stack and in some rare cases the value was NULL.

Now the value is initilized.

src/nxt_http_parse.c

@@ -1191,6 +1191,7 @@ nxt_http_fields_hash_collisions(nxt_lvlhsh_t *hash, nxt_mp_t *mp,
         }
 
         lhq.key_hash = nxt_http_field_hash_end(key) & mask;
+        lhq.value = &items[i];
 
         if (nxt_lvlhsh_insert(hash, &lhq) == NXT_DECLINED) {
             colls++;