thread.c: check type of coverage
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54075 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread.c: check type of coverage
only if non-zero.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54075 b2dd03c8-39d4-4d8f-98ff-823fe69b080e