io.c: truncate before appending

* 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/trunk@46372 b2dd03c8-39d4-4d8f-98ff-823fe69b080e