AddAttribute may take 'namespace:full:uri^attribute_name'
... e.g., created in SaxNamespaceFilter::startElement, which calls
XMLNamespaces::applyNSToAttributeName to make such a name.
See https://gerrit.libreoffice.org/c/core/+/145526/comments/26536db2_d8fb1e1b,
which notified that the assert from
f2f008c52aaa88329c07f441de60d6fdfce9f0b3
("Merge SvXMLAttributeList to comphelper::AttributeList", 2023-01-15) started
to fail on some input.
Change-Id: Ia0afc167aefac463bcd354dc009bff6e14a64dfa
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/146054
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>