configfile.c: Fixed a segfault after a parse error.

configfile.c: Fixed a segfault after a parse error.

In cf_read_generic(), the parse result had not been checked to not be
NULL, which caused a segfault when trying to access any of its members.
Now, an error will be returned in that case.

Also, cf_ci_append_children() has been made more robust in that respect.
It now detects an empty source and does nothing in that case.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>