iseq.c: remove duplicated strings for file paths

* iseq.c (iseq_location_setup): re-use existing string when iseq has
  the same path and absolute_path. [Bug #8149]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40334 b2dd03c8-39d4-4d8f-98ff-823fe69b080e