POSIX states that the preg parameter to regcomp() is undefined on
failure, so no need to call regfree() in these cases.

http://pubs.opengroup.org/onlinepubs/009695399/functions/regcomp.html

See also a discussion on the libvirt dev list

https://www.redhat.com/archives/libvir-list/2013-September/msg00262.html