Fix regression in the CVE-2009-3560 patch.

http://mail.libexpat.org/pipermail/expat-discuss/2009-December/002646.html

--- expat-2.0.1/lib/xmlparse.c.fix3560
+++ expat-2.0.1/lib/xmlparse.c
@@ -3703,6 +3703,9 @@ doProlog(XML_Parser parser,
         return XML_ERROR_UNCLOSED_TOKEN;
       case XML_TOK_PARTIAL_CHAR:
         return XML_ERROR_PARTIAL_CHAR;
+      case -XML_TOK_PROLOG_S:
+        tok = -tok;
+        break;
       case XML_TOK_NONE:
 #ifdef XML_DTD
         /* for internal PE NOT referenced between declarations */