linux-user: Check array bounds in errno conversion

Check array bounds in host_to_target_errno() and target_to_host_errno().

Signed-off-by: Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Message-id: 1441497448-32489-2-git-send-email-T.E.Baldwin99@members.leeds.ac.uk
[PMM: Add a lower-bound check, use braces on if(), tweak commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>