Unchecked device_add_child in iicsmb_attach: a failed smbus allocation leaves sc->smbus NULL and the next slave-mode interrupt dereferences it via smbus_intr(NULL,...)

Summary

The allocation is unchecked (contrast iicbb which returns ENXIO): with sc->smbus NULL, iicsmb_intr hands NULL into smbus_intr on both the INTR_STOP and INTR_ERROR paths -> device_get_softc(NULL) -> Fatal trap 12. ENOMEM-at-attach + a controller interrupt. Check-and-guard fix.