Xslt: fixed potential buffer overflow with null character.

Xslt: fixed potential buffer overflow with null character.

Due to shortcomings of the ccv->zero flag implementation in complex value
interface, length of the resulting string from ngx_http_complex_value()
might either not include terminating null character or include it,
so the only safe way to work with the result is to use it as a
null-terminated string.

Reported by Patrick Wollgast.