Set `summed_polns = 0` when `fb.sumifs == false` in sigproc_fb.c

[fxzjshm]

I'm still learning pulsar things, so please correct me if I'm wrong.

The default value of summed_polns is 1 in backend_common.c, so I think it need to be explicitly set to 0 here, so that readfile gives correct output ( affects this )

Also, the fb.ifstream is accessed in the next lines but seems never read from .fil files, which will affect s->poln_order and finally cause readfile's Polarization order = <some random stack memory data here>. But I cannot find some document about this header, so didn't change this.