iseq.c (rb_iseq_line_trace_each): explicit cast

Fix https://travis-ci.org/ruby/ruby/jobs/36814282
Tested with -Wconversion since my gcc lacks -Wshorten-64-to-32

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47760 b2dd03c8-39d4-4d8f-98ff-823fe69b080e