Fix uninitialized variables in udisks_daemon_log_valist()

Fix uninitialized variables in udisks_daemon_log_valist()

Assert that we don't get an undefined log level in udisks_daemon_log_valist().
This avoids falling through the switch and using uninitialized string
variables.

Fixes these warnings/errors:

cc1: warnings being treated as errors
udisksdaemon.c: In function ‘udisks_daemon_log’:
udisksdaemon.c:782:16: error: ‘level_str’ may be used uninitialized in this function
udisksdaemon.c:783:16: error: ‘level_color_str’ may be used uninitialized in this function
udisksdaemon.c:784:8: error: ‘syslog_priority’ may be used uninitialized in this function