string.c: potential memory leak

* string.c (rb_str_conv_enc_opts): allocate and hide wrapper before
  creating converter to be wrapped, to get rid of potential memory
  leak by failure on the allocation.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39856 b2dd03c8-39d4-4d8f-98ff-823fe69b080e