Fix extraction of namespace prefix from XML name.
Fixes CVE-2018-20843

This patch comes from upstream commit 11f8838bf99ea0a6f0b76f9760c43704d00c4ff6
https://github.com/libexpat/libexpat/commit/11f8838bf99ea0a6f0b76f9760c43704d00c4ff6

CVE is https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-20843

diff --git a/expat/lib/xmlparse.c b/expat/lib/xmlparse.c
index 30d55c5..737d7cd 100644
--- a/lib/xmlparse.c
+++ b/lib/xmlparse.c
@@ -6071,7 +6071,7 @@ setElementTypePrefix(XML_Parser parser, ELEMENT_TYPE *elementType)
       else
         poolDiscard(&dtd->pool);
       elementType->prefix = prefix;
-
+      break;
     }
   }
   return 1;