linux-user: Allow getsockopt() with NULL optval address

Some programs test availability of socket options by asking for the
value with an NULL optval address, which currenrly always trigger an
EFAULT in qemu.  Fix it by allowing a NULL address, in the same manner
as the Linux kernel on physical machines.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/2390
Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>