Fix conditional jump on uninitialized data in `dart_exit`

[bertwesarg]

Fixes valgrind error in dart_exit:

==28232== Conditional jmp or move depends on uninitialised value(s)
==28232==    at 0x43595B: dart__base__locality__domain__destruct (domain_locality.c:131)
==28232==    by 0x4393B9: dart__base__locality__delete (locality.c:225)
==28232==    by 0x439186: dart__base__locality__finalize (locality.c:83)
==28232==    by 0x432BAE: dart__mpi__locality_finalize (dart_locality_priv.c:37)
==28232==    by 0x4322EA: dart_exit (dart_initialization.c:251)
==28232==    by 0x4176ED: dash::finalize() (Init.cc:92)

[bertwesarg]

Somehow, we need to keep the domain_tag init to ".", even though dart__base__locality__domain__create_subdomains does this for us. If not, I get this runtime error:

[    0 ERROR ] [  1330 ] dart_locality.c          :53   !!! DART: dart_domain_team_locality: dart__base__locality__team_domain failed (3)

[devreal]

I think this one is for @fuchsto