Fix a bug with partial ProjectData and -c flag

[Alessio Vanni]

Hello,

With commit 77339944c (patch written by me), partial ProjectData
structures could be used to allow programs to be executed without
providing some files.

Unfortunately there's a bug that for some reason I managed to trigger
only now: under some conditions, using the -c flag generates an error
and aborts the execution.

The application that triggered it is a program that normally doesn't
require a configuration file, but can accept one to override the default
values if the user wishes to.

The attached patch should fix the issue.

Thanks,
A.V.

0001-fix-c-in-applications-without-base-config.patch
Description: Patch for GNUnet