st.c: re-calc hash_val before adding

* st.c (st_update): re-calculate hash_val before adding if key was
  changed, otherwise cannot access the newly added element if it
  has different hash value.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46722 b2dd03c8-39d4-4d8f-98ff-823fe69b080e