linux-user: make pwrite64/pread64(fd, NULL, 0, offset) return 0

linux-user: make pwrite64/pread64(fd, NULL, 0, offset) return 0

Linux returns success if pwrite64() or pread64() are called with a
zero length NULL buffer, but QEMU was returning -TARGET_EFAULT.

This is the same bug that we fixed in commit 58cfa6c2e6eb51b23cc9
for the write syscall, and long before that in 38d840e6790c29f59
for the read syscall.

Fixes: https://bugs.launchpad.net/qemu/+bug/1810433
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20190108184900[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>