syscall: fix dereference of undefined pointer

linux-user/syscall.c:5581:9: warning: Dereference of undefined pointer value
    if (*host_rt_dev_ptr != 0) {
        ^~~~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Suggested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>