fix(journal): treat missing PRIORITY field as unknown, not Emergency

[aki1770-del]

Problem

In get_journal_msg(), the PRIORITY field is read into buffer_priority and then passed to atoi():

dlt_system_journal_get(j, buffer_priority, "PRIORITY", sizeof(buffer_priority));
...
systemd_loglevel = atoi(buffer_priority);

When a journal entry has no PRIORITY field — common for auditd messages with _TRANSPORT=audit — dlt_system_journal_get() leaves buffer_priority as an empty string. atoi("") returns 0, which maps to severity level 0 (Emergency/Alert/Critical):

case 0:     /* Emergency */
case 1:     /* Alert */
case 2:     /* Critical */
    loglevel = DLT_LOG_FATAL;

All auditd log entries are thus emitted as DLT_LOG_FATAL with the label "Emergency:", regardless of their actual content.

Fixes #731.

Fix

Check buffer_priority[0] != '\0' before calling atoi(). Use -1 as a sentinel when the field is absent — -1 does not match any switch case, so the code falls through to default: loglevel = DLT_LOG_INFO, and the printed label shows prio_unknown: which accurately reflects the missing field.

systemd_loglevel = (buffer_priority[0] != '\0') ? atoi(buffer_priority) : -1;

Testing

Verified: auditd journal entries without PRIORITY are logged at DLT_LOG_INFO with label prio_unknown: after the fix.