Avoid uninitialized variable warnings with OBJECT_OFFSETOF() in MSVC.

Implementation of OBJECT_OFFSETOF() for non-GNUC compilers like MSVC
causes "uninitialized variable" warnings. Since OBJECT_OFFSETOF() is
indirectly used through all the *_FOR_EACH() (through ASSIGN_CONTAINER()
and  OBJECT_CONTAINING()) macros, the OVS build
on Windows gets littered with "uninitialized variable" warnings.
This patch attempts to workaround the problem.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>
Acked-by: Ben Pfaff <blp@nicira.com>