py-libxml2: work around a problem in error handling

Restore part of a patch lost in the last libxml2 update which is still
relevant. Reapplying it fixes segfaults caused by itstool, e.g., when
building editors/pluma, which is PR pkg/56229 from Andrius V.

Quoting from wiz@'s original commit from Jan 9, 2019, which covers
everything else:

"In some cases, invalid UTF-8 strings were returned which caused
Python interpreter crashes. See

itstool/itstool#22

Use a variant of the patch that was used in Fedora.

Bump PKGREVISION."

Fedora is still carrying this patch as-is.

(Also, evidently distinfo was not regenerated properly after the last
update, so there's a diff applied to it unrelated to this change set.)

(gutteridge)