pex-common.c (pex_read_err): Set stderr_pipe to -1 if a corresponding stream has been opened.

	* pex-common.c (pex_read_err): Set stderr_pipe to -1 if a
	corresponding stream has been opened.
	(pex_free): Close pipe file descriptors corresponding to child's
	stdout and stderr before waiting.

From-SVN: r163426