usb-ccid: make ccid_write_data_block() cope with null buffers

static code analyzer complain:

hw/usb/dev-smartcard-reader.c:816:5: warning: Null pointer passed as an argument to a 'nonnull' parameter
    memcpy(p->abData, data, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/usb/dev-smartcard-reader.c

@@ -813,7 +813,10 @@ static void ccid_write_data_block(USBCCIDState *s, uint8_t slot, uint8_t seq,
     if (p->b.bError) {
         DPRINTF(s, D_VERBOSE, "error %d\n", p->b.bError);
     }
-    memcpy(p->abData, data, len);
+    if (len) {
+        g_assert_nonnull(data);
+        memcpy(p->abData, data, len);
+    }
     ccid_reset_error_status(s);
     usb_wakeup(s->bulk, 0);
 }