linux-user: make bogus negative iovec lengths fail EINVAL

If the guest passes us a bogus negative length for an iovec, fail
EINVAL rather than proceeding blindly forward. This fixes some of
the error cases tests for readv and writev in the LTP.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
