merge revision(s) r46360,r46372: [Backport #8625]

	* io.c (io_setstrbuf, io_read): should not shorten the given buffer until
	  read succeeds.  [ruby-core:55951] [Bug #8625]

	* io.c (read_all): truncate the buffer before appending read data,
	  instead of truncating before reading.
	  [ruby-core:55951] [Bug #8625]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46629 b2dd03c8-39d4-4d8f-98ff-823fe69b080e