bn/asm/sparcv9-mont.pl: iron another glitch in squaring code path.

This module is used only with odd input lengths, i.e. not used in normal
PKI cases, on contemporary processors. The problem was "illuminated" by
fuzzing tests.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6440)

(cherry picked from commit f55ef97b)