Also account for switching away from "file"

Fixes #270.

url.bs

@@ -1490,14 +1490,17 @@ string <var>input</var>, optionally with a <a>base URL</a> <var>base</var>, opti
         <p>If <var>state override</var> is given, run these subsubsteps:
 
         <ol>
-         <li><p>If <var>url</var> <a>includes credentials</a> or has a non-null <a for=url>port</a>,
-         and <var>buffer</var> is "<code>file</code>", then return.
-
          <li><p>If <var>url</var>'s <a for=url>scheme</a> is a <a>special scheme</a> and
          <var>buffer</var> is not, then return.
 
          <li><p>If <var>url</var>'s <a for=url>scheme</a> is not a <a>special scheme</a> and
          <var>buffer</var> is, then return.
+
+         <li><p>If <var>url</var> <a>includes credentials</a> or has a non-null <a for=url>port</a>,
+         and <var>buffer</var> is "<code>file</code>", then return.
+
+         <li><p>If <var>url</var>'s <a for=url>scheme</a> is "<code>file</code>" and its
+         <a for=url>host</a> is an <a>empty host</a> or null, then return.
         </ol>
 
        <li><p>Set <var>url</var>'s <a for=url>scheme</a> to <var>buffer</var>.