gb18030 encoder: unwind from fourth byte when it's not a digit

Instead of always unwinding if there’s no code point when consuming the
fourth byte, only unwind when the fourth byte is not an ASCII digit.
This does mean that ASCII digits can be masked, but since ASCII digits
are not used as delimiter in any format this is highly unlikely to be
used in any attacks (and also matches existing implementations better).

Fixes #110.

Author: annevk

encoding.bs

@@ -1638,11 +1638,13 @@ consumers of content generated with <a>GBK</a>'s <a for=/>encoder</a>.
    <li><p>Set <a>gb18030 first</a>, <a>gb18030 second</a>, and
    <a>gb18030 third</a> to 0x00.
 
-   <li><p>If <var>code point</var> is null,
-   <a>prepend</a> <var>buffer</var> to
-   <var>stream</var> and return <a>error</a>.
+   <li><p>If <var>code point</var> is non-null, return a code point whose value is
+   <var>code point</var>.
 
-   <li><p>Return a code point whose value is <var>code point</var>.
+   <li><p>If <var>byte</var> is not in the range 0x30 to 0x39, inclusive, <a>prepend</a>
+   <var>buffer</var> to <var>stream</var>.
+
+   <li><p>Return <a>error</a>.
   </ol>
 
  <li>