Heap use-after-free in xmlSAX2AttributeNs

For https://bugzilla.gnome.org/show_bug.cgi?id=759020

* parser.c:
(xmlParseStartTag2): Attribute strings are only valid if the
base does not change, so add another check where the base may
change.  Make sure to set 'attvalue' to NULL after freeing it.
* result/errors/759020.xml: Added.
* result/errors/759020.xml.err: Added.
* result/errors/759020.xml.str: Added.
* test/errors/759020.xml: Added test case.