HID: bigbenff: prevent null pointer dereference

When emulating the device through uhid, there is a chance we don't have
output reports and so report_field is null.

Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20211202095334.14399-3-benjamin.tissoires@redhat.com