If we're passed garbage in malloc_init(), panic() rather than expecting

If we're passed garbage in malloc_init(), panic() rather than expecting
a KASSERT to handle it.  People are likely to turn off INVARIANTS RSN
and loading an old module can cause garbage-in here.

I saw the issue with an older nvidia driver (x11/nvidia-driver) loading
into a new kernel - a crash wasn't seen 'till sysctl_kern_malloc_stats().
I was lucky that mtp->ks_shortdesc was NULL and not something horrible.

While I'm here, KASSERT that malloc_uninit() isn't passed something that's
not in kmemstatistics.

MFC after:	3 weeks