filter: Force a minimal select timeout if something was converted.

When filter_preselect() runs. other pre_select functions might have
already been called and decided to do nothing, e.g. because their
output buffer was full or the input buffer was empty.

If this call to filter_preselect() changed any of the involved buffers,
we want those other pre_select() functions to be called ASAP. So force
a minimal timeout for the next select call to avoid unnecessary delays.