Fixed uninitialized memory read caused by stale regex captures.

Fixed uninitialized memory read caused by stale regex captures.

When ngx_http_regex_exec() reallocates r->captures array, it doesn't update
r->ncaptures value, if regex didn't match. So the next use of unnamed regex
capture triggers uninitialized read and potential buffer overrun.

This config demonstrates the issue:
    map test $my_map {
        volatile;

        ~mismatch(.*) 1; # reallocates r->captures in subrequests

        default "";
    }

    server {
        location ~(.*) { # sets r->ncaptures
            slice 50;

            # $1 will read from uninitialized memory in slice subrequests
            proxy_set_header Test $my_map$1;

            proxy_set_header Range $slice_range;
            proxy_pass http://backend;
        }
    }

The issue was introduced by 746fba0d79c6.