cpp: Fix handling of "no key" or "invalid time" situations
* lang/cpp/src/gpgaddexistingsubkeyeditinteractor.cpp (GpgAddExistingSubkeyEditInteractor::Private::nextState): Fix inverted logic of string comparisons.
This fixes the problem that the interactor didn't return the proper
error code if gpg didn't accept the key grip or the expiration date.
- GnuPG-bug-id: T6137