Prevent freeing a possible invalid pointer from journald

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

daemon/logger/journald/read.go

@@ -237,7 +237,10 @@ drain:
 
 	// free(NULL) is safe
 	C.free(unsafe.Pointer(oldCursor))
-	C.sd_journal_get_cursor(j, &cursor)
+	if C.sd_journal_get_cursor(j, &cursor) != 0 {
+		// ensure that we won't be freeing an address that's invalid
+		cursor = nil
+	}
 	return cursor
 }
 
@@ -272,7 +275,6 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 		s.readers.mu.Lock()
 		delete(s.readers.readers, logWatcher)
 		s.readers.mu.Unlock()
-		C.sd_journal_close(j)
 		close(logWatcher.Msg)
 	}()
 
@@ -307,9 +309,9 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 	following := false
 	defer func(pfollowing *bool) {
 		if !*pfollowing {
-			C.sd_journal_close(j)
 			close(logWatcher.Msg)
 		}
+		C.sd_journal_close(j)
 	}(&following)
 	// Remove limits on the size of data items that we'll retrieve.
 	rc = C.sd_journal_set_data_threshold(j, C.size_t(0))