...which violates the ctor's preconditions, so is UB.  Also, the "XML Help
Document Type Definition" appendix of
<http://documentation.openoffice.org/online_help/OOo2HelpAuthoring.pdf> states
that "branch" and "id" attributes are #REQUIRED for the "bookmark" element.

(There's probably further cases in the surrounding code where a std::string is
created from a potentially null argument, which would benefit from similar
fixes.)

Change-Id: I414576d13de784de1290951bcdd5e3ecb51f9cb8
Reviewed-on: https://gerrit.libreoffice.org/55735Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>