encoding.c: refill terminator at associating encoding

* encoding.c (rb_enc_associate_index): refill the terminator if it
  becomes longer than before.  [ruby-dev:47500] [Bug #8624]
* string.c (str_null_char, str_fill_term): get rid of out of bound
  access.
* string.c (rb_str_fill_terminator): add a parameter for the length of
  new terminator.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41930 b2dd03c8-39d4-4d8f-98ff-823fe69b080e