Fix multiline filter to not execute filter_matched unless it actually
matches.

1. A multiline filter was called and had a pattern that did not match
the event

2. If a pending event existed the current event was added to the pending
list, the old pending event was resurrected

3. The resurrected pending event wasn't cancelled so filter_matched()
was called even if the multiline did not, in fact, match

4. The resulting event object would have any tags added/removed and
fields added that the non-matching multiline cared to specify