Prevent redirect to URLs that begin with '///'

Visiting a logout URL like this:
    https://rp.example.co.jp/mellon/logout?ReturnTo=///fishing-site.example.com/logout.html
would have redirected the user to fishing-site.example.com

With the patch, this URL would be rejected.

Fixes: CVE-2021-3639

auth_mellon_util.c

@@ -927,6 +927,10 @@ int am_check_url(request_rec *r, const char *url)
 {
     const char *i;
 
+    if (url == NULL) {
+        return HTTP_BAD_REQUEST;
+    }
+
     for (i = url; *i; i++) {
         if (*i >= 0 && *i < ' ') {
             /* Deny all control-characters. */
@@ -943,6 +947,12 @@ int am_check_url(request_rec *r, const char *url)
         }
     }
 
+    if (strstr(url, "///") == url) {
+        AM_LOG_RERROR(APLOG_MARK, APLOG_ERR, HTTP_BAD_REQUEST, r,
+                          "URL starts with '///'");
+        return HTTP_BAD_REQUEST;
+    }
+
     return OK;
 }