string.c: add internal API rb_str_locktmp_ensure().
io.c (io_fread): use rb_str_locktmp_ensure(). [ruby-core:56121] [Bug #8669]
test/ruby/test_io.rb: add a test for above.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42212 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: add internal API rb_str_locktmp_ensure().
io.c (io_fread): use rb_str_locktmp_ensure().
[ruby-core:56121] [Bug #8669]
test/ruby/test_io.rb: add a test for above.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42212 b2dd03c8-39d4-4d8f-98ff-823fe69b080e