merge revision(s) 42212,42214: [Backport #8669]

	* 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.

	* io.c (io_getpartial): use rb_str_locktmp_ensure().
	  [ruby-core:56121] [Bug #8669]

	* io.c (rb_io_sysread): ditto.

	* test/ruby/test_io.rb: add tests for above.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@42216 b2dd03c8-39d4-4d8f-98ff-823fe69b080e