sys: don't panic on ifm_status being NULL

Some ethernet drivers that use miibus aren't probe/attaching the
miibus early enough for the ifioctl path to correctly run without
panicing.

Although yes, the drivers do need fixing, this stops them from
immediately panicing the kernel when they're loaded.

PR: kern/286530
Differential Revision: https://reviews.freebsd.org/D50136
Reviewed by: jhb