Fix potential html injection when the database server isn't available (bug 26164).

Fix potential html injection when the database server isn't available (bug 26164).

Wikis which set $wgServer in their LocalSettings or are in a virtual
host would never be vulnerable.
For sites which show the wiki in the default host, it will depend on how
forgiving is their webserver and php stack for that garbled input, although
some kind of foolable proxy —moreover wrongly caching errors (or the default
output buffering is disabled and something incorrectly sent a previous text)—
would also need to be present in order to make that useful for a
potential attacker.