pty.c: more difensive
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64766 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
pty.c: more difensive
[ruby-core:89043] [Bug #15116]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64766 b2dd03c8-39d4-4d8f-98ff-823fe69b080e