If the value of get_image_size() exceeds INT_MAX / 2 - 10000, the
computation of @dt_size overflows to a negative number, which then
gets converted to a very large size_t for g_malloc0() and
load_image_size().  In the (fortunately improbable) case g_malloc0()
succeeds and load_image_size() survives, we'd assign the negative
number to *sizep.  What that would do to the callers I can't say, but
it's unlikely to be good.

Fix by rejecting images whose size would overflow.

Reported-by: Kurtis Miller <kurtis.miller@nccgroup.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190409174018.25798-1-armbru@redhat.com>