lib/open3.rb: make sure that pipes are closed

Attempting to invoke a non-existent command led to the leak of fds.

http://rubyci.s3.amazonaws.com/graviton2/ruby-master/log/20200526T140004Z.log.html.gz
```
[ 9101/20195] TestGemExtCmakeBuilder#test_self_build = 0.01 sLeaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 7 : #<IO:fd 7>
Leaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 10 : #<IO:fd 10>
Leaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 11 : #<IO:fd 11>
Leaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 12 : #<IO:fd 12>
```

0 files changed, 0 insertions, 0 deletions