connection: Prevent integer overflow in DIV_ROUNDUP.

The DIV_ROUNDUP macro would overflow when trying to round values higher
than MAX_UINT32 - (a - 1). The result is 0 after the division. This is
potential security issue when demarshalling an array because the length
check is performed with the overflowed value, but then the original huge
value is stored for later use.

The issue was present only on 32bit platforms. The use of size_t in the
DIV_ROUNDUP macro already promoted everything to 64 bit size on 64 bit
systems.
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>

Style changes by Derek Foreman