linux-user/syscall.c: lock_iovec: unlock vec[i] in failure processing code block

When failure occurs during locking of vec[i], we also need to unlock all
already locked vec[i] in failure processing code block before return.

Code in unlock_user() checks vec[i].iov_base for NULL, so there's no
need not check it .

If error is EFAULT when "i == 0", vec[i].iov_base is NULL, we can just
skip it, so can still use "while (--i >= 0)" loop condition.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
