linux-user: fix signal number range check

When translating between host and target signal numbers keep negative
numbers unchanged, avoiding access beyond array bounds.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 878v2b8sek.fsf@igel.home
