sink-input: Fix underrun_for calculation when resampling.
pa_sink_input_seek() calculates output lenth (slength) and
corresponding input length (ilength). During an underrun, the function
generates slength bytes of silence and adds ilength to the
underrun_for value. However, the ilength value may be shortened to
match resampler limits, and there's no corresponding adjustment to
slength. Thus, the length of the generated silence is longer than
resampler output would have been, and underrun_for should be increased
by more than the limited ilength. This error makes the user-visible
since_underrun field in struct pa_timing_info too small. Fix by using
the original value calculated before limiting in this case.