The recent change to initialization of blists (r324420) relied on '-1'

The recent change to initialization of blists (r324420) relied on '-1'
appearing only where the code explicitly set it, but since much of the
data was not initialized, '-1' appeared other places too, and led to
panics.  Clear the allocated data before initializing nonzero values by
allocating with M_ZERO.

Submitted by:	Doug Moore <dougm@rice.edu>
Reported by:	Oleg V. Nauman <oleg@theweb.org.ua>, cy
Tested by:	Oleg V. Nauman <oleg@theweb.org.ua>
MFC after:	1 week
X-MFC with:	r324420
Differential Revision:	https://reviews.freebsd.org/D12627