harden REALLOC_ARRAY and xcalloc against size_t overflow

harden REALLOC_ARRAY and xcalloc against size_t overflow

REALLOC_ARRAY inherently involves a multiplication which can
overflow size_t, resulting in a much smaller buffer than we
think we've allocated. We can easily harden it by using
st_mult() to check for overflow.  Likewise, we can add
ALLOC_ARRAY to do the same thing for xmalloc calls.

xcalloc() should already be fine, because it takes the two
factors separately, assuming the system calloc actually
checks for overflow. However, before we even hit the system
calloc(), we do our memory_limit_check, which involves a
multiplication. Let's check for overflow ourselves so that
this limit cannot be bypassed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>