There was a race where threads were themselves responsible for increasing the
counter for the number of running threads. This left an open window where the
thread was already started, but our counter was not yet increased. The effect of
this race would be that we start more threads than we should.
Fix this by increasing the counter before actually starting new worker threads.
Signed-off-by: Uli Schlachter <psychon@znc.in>
0fddbba230