Plug resource leak.

Found with:	Coverity Prevent
CID:		7040

usr.sbin/chown/chown.c

@@ -290,6 +290,7 @@ chownerr(const char *file)
 			err(1, "malloc");
 		ngroups = getgroups(ngroups_max, groups);
 		while (--ngroups >= 0 && gid != groups[ngroups]);
+		free(groups);
 		if (ngroups < 0) {
 			warnx("you are not a member of group %s", gname);
 			return;