dbm.c: yield dup of keystr

* ext/dbm/dbm.c (fdbm_fetch): yield dup of keystr, to make it shared
  and get rid of use of uninitialized variable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44482 b2dd03c8-39d4-4d8f-98ff-823fe69b080e