Perl: request object validation

Perl: request object validation

Previously, using stale request objects resulted in accesses to already
freed memory, causing segmentation faults:

    location /stale {
        perl 'sub {
            my $r = shift;
            $prev->log_error(0, "next request arrived") if $prev;
            $prev = $r;
            $r->send_http_header;
            return OK;
        }';
    }

Similarly, incorrectly blessed objects might cause segmentation faults,
such as in the following configuration:

    location /bless {
        perl 'sub {
            my $v = 10;
            my $r = bless \$v, "nginx";
            $r->send_http_header;
            return OK;
        }';
    }

With this change, active request object is recorded in the
ngx_http_perl_call_handler() function, and checked by
ngx_http_perl_set_request() to prevent use of unexpected request
objects.

Reported by Axel Mierczuk, Keith Hoodlet, 1Password’s Off-by-1 Labs.

Signed-off-by: Sergey Kandaurov <pluknet@nginx.com>
Origin: https://freenginx.org/hg/nginx/rev/86a2685756ae