tests: Fix callback arguments in fake-document-portal

They didn’t match the prototype generated by gdbus-codegen, which meant that the FD list was being iterated incorrectly. Secondly, the document ID list returned by the method was not NULL terminated, which could lead to reading off the end of the list.

Somehow, neither of these bugs caused problems on Linux, but they did cause problems on FreeBSD.

Signed-off-by: Philip Withnall withnall@endlessm.com

Fixes: #1983 (closed)