logger: fix hardfault for invalid SDLOG_PROFILE setting
logger: fix hardfault for invalid SDLOG_PROFILE setting

If some bits are set but no topic was added, _subscriptions was null but
later accessed.

During normal use this only happens when switching between different
firmware versions with different SDLOG_PROFILE definition (with custom
config).

krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware