linux-user: Fix Coverity CID 1430271 / CID 1430272

linux-user: Fix Coverity CID 1430271 / CID 1430272

In new functions print_ioctl() and print_syscall_ret_ioctl(), we don't
check if lock_user() returns NULL and this would cause a segfault in
thunk_print().

If lock_user() returns NULL don't call thunk_print() but prints only the
value of the (invalid) pointer.

Tested with:

    # cat ioctl.c
    #include <unistd.h>
    #include <sys/ioctl.h>

    int main(void)
    {
        int ret;

        ret = ioctl(STDOUT_FILENO, TCGETS, 0xdeadbeef);
        ret = ioctl(STDOUT_FILENO, TCSETSF, 0xdeadbeef);
        return 0;
    }
    # QEMU_STRACE= ./ioctl
    ...
    578 ioctl(1,TCGETS,0xdeadbeef) = -1 errno=2 (Bad address)
    578 ioctl(1,TCSETSF,0xdeadbeef) = -1 errno=2 (Bad address)
    ...
    # QEMU_STRACE= passwd
    ...
    623 ioctl(0,TCGETS,0x3fffed04) = 0 ({})
    623 ioctl(0,TCSETSF,{}) = 0
    ...

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 79482e5987c8 ("linux-user: Add strace support for printing arguments of ioctl()")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>