fdo#86169 fix completion of function name crash

function names with dot or underscore were not completed
correctly, leading to incorrect results or even crashes

Change-Id: Iacb6af29c609ab39746185c71d4443537bf46b5e
Reviewed-on: https://gerrit.libreoffice.org/12380
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>