merge revision(s) r45979: [Backport #9847]

	* io.c (io_setstrbuf): always check if the buffer is modifiable.
	  [ruby-core:62643] [Bug #9847]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46614 b2dd03c8-39d4-4d8f-98ff-823fe69b080e