Deal with blob: URLs that have no backing blob

Tests: web-platform-tests/wpt#33941.

Fixes #1077.

Author: dlrobertson

fetch.bs

@@ -4503,12 +4503,14 @@ steps:
     <a for="fetch params">canceled</a>:
 
     <ol>
-     <li><p>Let <var>blob</var> be <var>request</var>'s <a for=request>current URL</a>'s
-     <a for=url>blob URL entry</a>'s <a for="blob URL entry">object</a>.
+     <li><p>Let <var>blobURLEntry</var> be <var>request</var>'s <a for=request>current URL</a>'s
+     <a for=url>blob URL entry</a>.
 
      <li>
-      <p>If <var>request</var>'s <a for=request>method</a> is not `<code>GET</code>` or
-      <var>blob</var> is not a {{Blob}} object, then return a <a>network error</a>. [[!FILEAPI]]
+      <p>If <var>request</var>'s <a for=request>method</a> is not `<code>GET</code>`,
+      <var>blobURLEntry</var> is null, or <var>blobURLEntry</var>'s
+      <a for="blob URL entry">object</a> is not a {{Blob}} object, then return a
+      <a>network error</a>. [[!FILEAPI]]
 
       <p class=note>The `<code>GET</code>` <a for=/>method</a> restriction serves no useful purpose
       other than being interoperable.