ccid-card-emul: do not crash if backend is not provided

Program received signal SIGSEGV, Segmentation fault.
__strcmp_sse42 () at ../sysdeps/x86_64/multiarch/strcmp-sse42.S:164
164               movdqu        (%rsi), %xmm2
(gdb) bt
    at /home/elmarco/320g/src/qemu/hw/ccid-card-emulated.c:477
    at /home/elmarco/320g/src/qemu/hw/ccid-card-emulated.c:503

Signed-off-by: Marc-André Lureau <mlureau@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
