detect * with no character following as invalid format code

... so adding it to the list in Format Numbers using the checkmark
button is not possible.

Change-Id: I57ca6a7a3617583a82a99c87291373b1fe4e246d
(cherry picked from commit bf674ab0f3097ad122c727b4d86f004f8cce1f61)
Reviewed-on: https://gerrit.libreoffice.org/10998
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>