test-hashmap: check allocation computation for overflow

test-hashmap: check allocation computation for overflow

When we allocate the test_entry flex-struct, we have to add
up all of the elements that go into the flex array. If these
were to overflow a size_t, this would allocate a too-small
buffer, which we would then overflow in our memcpy steps.

Since this is just a test-helper, it probably doesn't matter
in practice, but we should model the correct technique by
using the st_add() macros.

Unfortunately, we cannot use the FLEX_ALLOC() macros here,
because we are stuffing two different buffers into a single
flex array.

While we're here, let's also swap out "malloc" for our
error-checking "xmalloc", and use the preferred
"sizeof(*var)" instead of "sizeof(type)".

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