journalctl: always initialize global variables

That's not necessary, as they are initialized with zero, but for safety
and readability.

(cherry picked from commit 4f0165f)

src/journal/journalctl.c

@@ -112,7 +112,7 @@ static char *arg_verify_key = NULL;
 static usec_t arg_interval = DEFAULT_FSS_INTERVAL_USEC;
 static bool arg_force = false;
 #endif
-static usec_t arg_since, arg_until;
+static usec_t arg_since = 0, arg_until = 0;
 static bool arg_since_set = false, arg_until_set = false;
 static char **arg_syslog_identifier = NULL;
 static char **arg_system_units = NULL;