Fix v constructor to always initialize its state.

Depending on how structure containing v is initialized
runid is intialized or not, which leads memcheck to complain:

Conditional jump or move depends on uninitialised value(s)
   at 0x4E5F3CD: pmemobj_volatile (sync.c:656)
   by 0x10AF36: pmem::obj::v<(anonymous namespace)::foo>::get() (v.hpp:125)
   by 0x10A830: (anonymous namespace)::test_init(pmem::obj::pool<(anonymous namespace)::root>&) (v.cpp:80)
   by 0x10AAC8: main (v.cpp:105)