Hash initialization ignores elements with key.data set to NULL.
Nevertheless, the initial hash bucket size check didn't skip them,
resulting in unnecessary restrictions on, for example, variables with
long names and with the NGX_HTTP_VARIABLE_NOHASH flag.

Fix is to update the initial hash bucket size check to skip elements
with key.data set to NULL, similarly to how it is done in other parts
of the code.

Found by Coverity (CID 1451664).

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().

A configuration like

    server { server_name .foo^@; }
    server { server_name .foo; }

resulted in a segmentation fault during construction of server names hash.

Reported by Markus Linnala.
Found with afl-fuzz.

Found by Clang Static Analyzer.

Initial size as calculated from the number of elements may be bigger
than max_size.  If this happens, make sure to set size to max_size.

Reported by Chris West.

Previously, maximum size of a hash table built was (max_size - 1).