spwaterfall: fixing check for setting commands to NULL
spwaterfall: fixing check for setting commands to NULL

jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,wangning223/liquid-dsp,wangning223/liquid-dsp,wangning223/liquid-dsp,jgaeddert/liquid-dsp,wangning223/liquid-dsp,wangning223/liquid-dsp,jgaeddert/liquid-dsp