When UniscribeLayout::LayoutText(ImplLayoutArgs & rArgs) is run with
BiDiRtl flag, and some skipped VisualItems was reversed in lines 908-927,
it's possible that they will arrive in reverse order to the code adjusting
mnSubStringMin, so this assertion may fire.

This situation is not erroneous, so simply replacing the assert with "if".

Change-Id: Icb244502dd00b4c08a2c5192369ee66d743b0dcc
Reviewed-on: https://gerrit.libreoffice.org/19835Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>