For https://bugzilla.gnome.org/show_bug.cgi?id=734280

libxml2 reports wrong error column numbers (field int2 in xmlError)
in structured error handler, after parsing XML attribute values.

Example XML:

<?xml version="1.0" encoding="UTF-8"?>
<root
xmlns="urn:colbug">&</root>
<!--
         1         2         3         4
1234567890123456789012345678901234567890
-->

Expected location of the error would be line 3, column 21.

The actual location of the error is line 3, column 9:

$ ./xmlparse colbug2.xml
colbug2.xml:3:9: xmlParseEntityRef: no name

The 12 characters of the xmlns attribute value "urn:colbug" are
not accounted for in the error column value.