Fix CSV output containing increasing number of null byte characters.

The CSV buffer is being truncated on each new row, but since the
stream's pointer isn't also being reset, each new row starts at the same
position as the previous row, but with null bytes up until that point.
This leads to increasing growth in the length of each CSV row, since
each line will be padded with null bytes equivalent to the previous
row's length.

linkcheck/logger/csvlog.py

@@ -128,6 +128,7 @@ class CSVLogger (_Logger):
         # ... and write to the target stream
         self.write(data)
         # empty queue
+        self.queue.seek(0)
         self.queue.truncate(0)
 
     def end_output (self, **kwargs):