Use xcalloc instead of xmalloc with calculated sizes

Ensure we don't have integer overflow when allocating the array
of struct iovec for store commands.

Change-Id: I0777ca8ef050b9773dba0c03b5e8533d3b2d5486
Reviewed-on: https://gerrit.stanford.edu/1560
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>