string.c: fix for UTF-16/32

* string.c (rb_str_inspect): get rid of out-of-bound access.
* string.c (rb_str_inspect): when a UTF-16/32 string doesn't have a
  BOM, inspect as a dummy encoding string.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43035 b2dd03c8-39d4-4d8f-98ff-823fe69b080e