gc.c: use size_t and no header if next_index == 0

* gc.c (gc_profile_dump_on): use size_t to get rid of overflow and
  show the header when next_index > 0, instead of next_index != 1.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40889 b2dd03c8-39d4-4d8f-98ff-823fe69b080e