time.c: check re-initialize

* time.c (GetTimeval): check if already initialized instance.
* time.c (GetNewTimeval): check if newly created instance.
* time.c (time_init_0, time_init_1, time_init_copy, time_mload): must
  be newly created instance.  [ruby-core:53436] [Bug #8099]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39766 b2dd03c8-39d4-4d8f-98ff-823fe69b080e