Logging: avoid "cb was NULL" warning message during logging shutdown

Logging: avoid "cb was NULL" warning message during logging shutdown

Commit 4b9234088f56 introduced the fetch and restore of the libraries'
default log routines in addition to the registration of a Pulseview
application's log routine. Some code paths in main() could result in
unexpected "cb was NULL" messages on stderr, when logging::init() did
not execute but the destructor tried to restore a handler which was not
fetched before. Silence the error message.