Core: fixed segfault with too large bucket sizes (ticket #1806). To save memory hash code uses u_short to store resulting bucket sizes, so maximum bucket size is limited to 65536 minus ngx_cacheline_size (larger values will be aligned to 65536 which will overflow u_short). However, there were no checks to enforce this, and using larger bucket sizes resulted in overflows and segmentation faults. Appropriate safety checks to enforce this added to ngx_hash_init().