Prevent crash when get_pmt is called but the PMT is not initialized.
Prevent crash when new service is enabled but the PMT is not configured.
Remove only valid config entries on exit.

This prevents crash on exit when a config entry was removed while
dvblast was running and the configration was reloaded.
Enable b_select_pmts when configuration file is used.

When configuration file is used it is reasonable to assume that
services may be added/removed on the fly.

If b_select_pmts is not set, dvblast is unable to start streaming
newly added services which is not expected at all.