BUG/MINOR: errors: startup_logs_free: set global startup_logs ptr to NULL

ring_free() calls free() on the ring struct pointer, but startup_logs continues
to keep this address. So let's reset at the end startup_logs to NULL.
startup_logs is checked in print_message().

No need to backport this fix, as it's related to the latest master-worker
refactoring.

src/errors.c

@@ -150,6 +150,7 @@ void startup_logs_free(struct ring *r)
 		munmap(ring_allocated_area(r), STARTUP_LOG_SIZE);
 #endif /* ! USE_SHM_OPEN */
 	ring_free(r);
+	startup_logs = NULL;
 }
 
 /* duplicate a startup logs which was previously allocated in a shm */