Fix XSS caused by invalid utf8.

[evazion]

Here's a rather interesting bug I found with the the help of american fuzzy lop. Stick the following into a view and observe an XSS:

  <%= format_text("[[\xFA<script \xFA>alert(document.cookie); //\xFA</script \xFA>]]", :ragel => true) %>

@user\xF4<b>xss\xFA</b> and "url":/page\xF4">x\xFA<b>xss\xFA</b> are similar exploits.

The issue is that g_markup_escape_text is used for escaping html, but this function assumes the input is valid utf8. When given invalid utf8 it goes haywire. Specifically, \xFA is an invalid leading byte, which seems to cause the function to copy the next few bytes unescaped until it resynchronizes with the utf8 stream.

The fix I went with is to raise an exception if the input isn't valid utf8. I also removed some stuff that tries to handle nulls since g_validate_utf8 forbids those too. AFAICT the purpose of this code was to close any unclosed tags when EOF is reached, but that's redundant since parse_helper calls dstack_close anyway.

In practice, postgres doesn't allow text fields to contain invalid utf8 or nulls to begin with, so in the real world this doesn't seem possible to trigger.