Fix redirect URL validation bypass

It turns out that browsers silently convert backslash characters into
forward slashes, while apr_uri_parse() does not.

This mismatch allows an attacker to bypass the redirect URL validation
by using an URL like:

  https://sp.example.org/mellon/logout?ReturnTo=https:%5c%5cmalicious.example.org/

mod_auth_mellon will assume that it is a relative URL and allow the
request to pass through, while the browsers will use it as an absolute
url and redirect to https://malicious.example.org/ .

This patch fixes this issue by rejecting all redirect URLs with
backslashes.
void am_strip_blank(const char **s)
{
    while ((**s == ' ') || (**s == '\t'))
        (*s)++;
    return;
}

void am_strip_blank(const char **s)
{
    while ((**s == ' ') || (**s == '\t'))
        (*s)++;
    return;
}
