dds: fixing pointer initialization issue with 0 stages
dds: fixing pointer initialization issue with 0 stages

  * a zero-stage DDS is essentially just a mixer
  * this is a valid configuration, but the pointers for buffering
    were not being initialized properly
  * NULL pointer was being dereferenced causing a seg fault
  * pointers are now set to proper values without running through loop

jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp