Avoid a double-free on the extra drainJournal call

The drainJournal() function frees a passed-in cursor and returns a new
one.  The final call we made to drainJournal() in followJournal()
mistakenly discarded the updated cursor value, which would likely
produce a double-free error.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Author: nalind

daemon/logger/journald/read.go

@@ -264,7 +264,7 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 			// exited. Try to drain the journal one more time to pick up any last-minute journal entries.
 			// Note, this isn't fool-proof and there's no guarantee that we'll get all the trailing
 			// entries, but this is better than nothing, as it does yield entries more often than not.
-			s.drainJournal(logWatcher, config, j, cursor)
+			cursor = s.drainJournal(logWatcher, config, j, cursor)
 		}
 		// Clean up.
 		C.close(pfd[0])