mesh: Fix regression error in HB subscription set

mesh: Fix regression error in HB subscription set

This fixes a regression introduced in commit c77bb848a9fb
("mesh: Refactor heartbeat pub/sub"):
Heartbeat subscription should be enabled unless either source or
destination are unassigned addresses or period log is zero.

Also, decrement ref count on group destination address if period log
is zero since the subscription is not supposed to be processed.

Change-Id: I981784d5e260e90dce022ee8785d3d1b47348e57
Signed-off-by: anuj.bhumiya <anuj.bhumiya@samsung.com>