Script: avoid garbage at the end of the result string

Script: avoid garbage at the end of the result string

If the script result turned out to be shorter than its predicted
length, the result string contained uninitialized bytes at the end.
The fix is to cut the result string by its actual size.

The following locations returned trailing garbage to the client with
URI "/1234abcd".

    map $uri $foo {
        ~^/(?<bar>[0-9]).*$ $bar;
    }

    location ~(?<bar>[0-9]*)[a-z]*$ {
        return 200 $1:$foo;
    }

    location ~(?<bar>[0-9]*)[a-z]*$ {
        set $qux $1:$foo;
        return 200 $qux;
    }