do not infloop on a query ending in %XY, for invalid hex X or Y

When a query ends in say %gg, (or any invalid hex) e.g.,
http://git.gnome.org/browse/gdlmm/commit/?id=%gg
convert_query_hexchar calls memmove(txt, txt+3, 0), and then returns
txt-1, so the loop in http_parse_querystring never terminates.  The
solution is to make the memmove also copy the trailing NUL.
* html.c (convert_query_hexchar): Fix off-by-one error.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>