check if buf/bufsz are non-NULL before freeing.

not all control messages that can be received result in buf being
initialized, we might get a spurious different control message

seen in practice when swapping modes a few times on a urndis device

urndis0: SAMSUNG ...
autoconfiguration error: urndis0: invalid address
panic: kernel diagnostic assertion "p != NULL" failed: file "/cvs/src/sys/kern/subr_kmem.c", line 263
cpu0: Begin traceback...
vpanic() at netbsd:vpanic+0x178
kern_assert() at netbsd:kern_assert+0x48
kmem_intr_free() at netbsd:kmem_intr_free+0xca
urndis_attach() at netbsd:urndis_attach+0x4c2

(maya)