clear allocated blist struct

[dougm]

Summary

The recent change to initialization of blists 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 set in the last malloc argument.

[cy]

This fixes the recurring panic on my gateway machine.