[Process][2.1] Do not reset stdout/stderr pipes on stream_select failure

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
License of the code: MIT

When `stream_select` is called and failed, pipes are currently reset to an empty array. As a result, both error-output and output are empty whereas the exitcode is 0 (successful process).

There is no reason (at least I don't see it) to reset pipes when an error occurs.