libiberty: fix memory leak in pex-win32.c and refactor

Fix memory leak of cmdline buffer and refactor to have
cleanup code appear once for all exit cases.

libiberty/ChangeLog:

	* pex-win32.c (win32_spawn): Fix memory leak of cmdline
	buffer and refactor to have cleanup code appear once
	for all exit cases.

Signed-off-by: Jonathan Yong <10walls@gmail.com>