IoCtxImpl: initialize last_objver with 0

CID 717219 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "last_objver" is not
 initialized in this constructor nor in any functions that it calls.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>