sbr/vector.c: Zero the growth with memset(3), not loop.

When the resize functions grow the vectors, as they always do, zero the
new slots with memset(3) rather than a for loop.  Particularly of note
for the bit vector where it was bvector_clear()ing one bit at a time.
Although a NULL pointer needn't have a representation of all-zero bits,
the code was already assuming that, e.g. on the initial allocation, so
we're no worse off.