posix: Fix pipe fd leak if redirecting both stdout and stderr

This re-adds the close(source) call which already existed here before
commit caa7b2f. pipe_out already closes the source fd when redirecting
stdout or stderr, it was just missing when redirecting both at once.

The pipe fds must be closed after redirecting to avoid keeping the pipe
open unnecessarily, for example the parent may want to close their end of
the pipe.

Closes: #353
Signed-off-by: Daniel Klauer <[email protected]>

include/boost/process/detail/posix/pipe_out.hpp

@@ -91,6 +91,7 @@ void pipe_out<1,2>::on_exec_setup(Executor &e) const
          e.set_error(::boost::process::detail::get_last_error(), "dup2() failed");
     if ((sink != STDOUT_FILENO) && (sink != STDERR_FILENO))
         ::close(sink);
+    ::close(source);
 }
 
 class async_pipe;