Limit req: improved error handling when parsing "zone" parameter of

Limit req: improved error handling when parsing "zone" parameter of
"limit_req_zone" directive; minimum size of zone is increased.

Previously an unsigned variable was used to keep the return value of
ngx_parse_size() function, which led to an incorrect zone size if NGX_ERROR
was returned.

The new code has been taken from the "limit_conn_zone" directive.