Correct the condition under which we allocate a terminator node.

Correct the condition under which we allocate a terminator node.

We will have last_block < blocks if the block count is divisible
by BLIST_BMAP_RADIX, but a terminator node is still needed if the
tree isn't balanced.  In this case we were overruning the blist
array by 16 bytes during initialization.

While here, add a check for the invalid blocks == 0 case.

PR:		231116
Reviewed by:	alc, kib (previous version), Doug Moore <dougm@rice.edu>
Approved by:	re (gjb)
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17020