Fix up potential overflows in size calculations by replacing

Fix up potential overflows in size calculations by replacing
instances of pool_alloc(p, x * sizeof *y) by pool_malloc(p, x,
sizeof *y) everywhere I could find them.  Similarly by
pool_malloc(), pool_realloc().
(Order is important: pool_alloc(p, sizeof *y, x) will divide by 0
if x is 0.)