Fixed division by zero exception in ngx_hash_init().

The ngx_hash_init() function did not expect call with zero elements count,
which caused FPE error on configs with an empty "types" block in http context
and "types_hash_max_size" > 10000.

Example configuration to reproduce:

  events { }

  http {
          types_hash_max_size 10001;
          types {}
          server {}
  }