backup: do not try to read tar's stderr past EOF

If EOF is reached on tar's stderr, stop reading it, even if didn't found
expected data. Log this event.

This may happen when tar output some fatal error, instead of filelist.

qubesadmin/backup/restore.py

@@ -466,6 +466,9 @@ class ExtractWorker3(Process):
             # yet, so it will hang on write() when the output pipe fill up).
             while True:
                 line = tar2_process.stderr.readline()
+                if not line:
+                    self.log.warning('EOF from tar before got file size info')
+                    break
                 line = line.decode()
                 if _tar_msg_re.match(line):
                     self.log.debug('tar2_stderr: %s', line)