string.c: rb_str_cat_cstr

* string.c (rb_str_cat): make non-buf version main.

* string.c (rb_str_cat_cstr): rename from rb_str_cat2.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45609 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: argument check

* string.c (rb_str_cat_cstr): check the argument as other `_cstr`
  functions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45610 b2dd03c8-39d4-4d8f-98ff-823fe69b080e