linux-user: fix "may be used uninitialized" warnings

linux-user: fix "may be used uninitialized" warnings

Fixes:

/home/elmarco/src/qemu/linux-user/syscall.c: In function ‘do_ioctl_rt’:
/home/elmarco/src/qemu/linux-user/syscall.c:4773:9: error: ‘host_rt_dev_ptr’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     if (*host_rt_dev_ptr != 0) {
         ^~~~~~~~~~~~~~~~
/home/elmarco/src/qemu/linux-user/syscall.c:4774:9: error: ‘target_rt_dev_ptr’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
         unlock_user((void *)*host_rt_dev_ptr,
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                     *target_rt_dev_ptr, 0);
                     ~~~~~~~~~~~~~~~~~~~~~~

Based on previous discussion from patch "linux-users/syscall: make
do_ioctl_rt safer" by Alex Bennée.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20190305151500.25038-1-marcandre.lureau@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>