Parameter count is size byte, so.. SUM(1,1,1,...) with 256 arguments resulted
in 0 (uint8 wrapping around).

(cherry picked from commit 209cc5c2)

Change-Id: Ib9997ad0d0d13d4c5171f276148b6c5cad570d5b
Reviewed-on: https://gerrit.libreoffice.org/39506Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
(cherry picked from commit 1d1bdfd9)