nIcuErr after the second RegexMatcher::find() call was
U_STRING_NOT_TERMINATED_WARNING hence a check for U_ZERO_ERROR failed
and a content string of "foo <emph>bar</emph> baz" became
"foo <emph>bar&lt;/emph&gt; baz" which later provoked a missing ending
tag error.

This should had used U_SUCCESS(nIcuErr) instead and only worked by
chance. Apparently the warning wasn't issued in earlier ICU versions and
only introduced with 55 or 56.

Change-Id: I36482c928544fc6e8c0403997af33084a47d4bea
(cherry picked from commit 84017530)
Reviewed-on: https://gerrit.libreoffice.org/19524Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit a3aec6c5)