Don't fatal when encountering [[ <bad URI > | ... ]]

Summary:
Fixes T12796. In D17647, the parser became more strict, but this remarkup rule doesn't deal with it gracefully.

Instead, detect when the parse failed and bail out.

Test Plan:
  - Put `[[ http://good.com#u:p@evil.com/ | broken ]]` into a Remarkup document without backticks.
  - Before patch: fatal ("rejecting ambiguous URI").
  - After patch: link doesn't work (which is correct), but page does.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12796

Differential Revision: https://secure.phabricator.com/D18076

src/markup/engine/remarkup/markuprule/PhutilRemarkupDocumentLinkRule.php

@@ -120,9 +120,16 @@ public function markupDocumentLink(array $matches) {
       $protocols = $this->getEngine()->getConfig(
         'uri.allowed-protocols',
         array());
-      $protocol = id(new PhutilURI($uri))->getProtocol();
-      if (!idx($protocols, $protocol)) {
-        // Don't treat this as a URI if it's not an allowed protocol.
+
+      try {
+        $protocol = id(new PhutilURI($uri))->getProtocol();
+        if (!idx($protocols, $protocol)) {
+          // Don't treat this as a URI if it's not an allowed protocol.
+          $is_uri = false;
+        }
+      } catch (Exception $ex) {
+        // We can end up here if we try to parse an ambiguous URI, see
+        // T12796.
         $is_uri = false;
       }
     }