getrandom(2) and read(2) (from /dev/urandom) may return a random buffer
whose length is shorter than required.  This change makes sure that they
get enough buffer by using a loop.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61292 b2dd03c8-39d4-4d8f-98ff-823fe69b080e