Core: fixed segfault with too large bucket sizes (ticket #1806).

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().