Unknown (match tags="") first in rule messes up regular flushing

[unhammer]

with a rule like

<rule weight="2.0">
    <match tags=""/>
    <match lemma="fremholde" tags="vblex.*"><select lemma="peike# på"/></match>
  </rule>

, the input buffer doesn't seem to be emptied until EOF. I can pipe a corpus through and see no output until the whole corpus has been read (and then everything comes at once). Remove the <match tags=""/> from the start of the rule and everything is fine again.

cf #39 / #40