Filter: fixed crash on zero cutoff frequency

this fixes #10435

libraries/Filter/LowPassFilter2p.cpp

@@ -35,6 +35,10 @@ template <class T>
 void DigitalBiquadFilter<T>::compute_params(float sample_freq, float cutoff_freq, biquad_params &ret) {
     ret.cutoff_freq = cutoff_freq;
     ret.sample_freq = sample_freq;
+    if (!is_positive(ret.cutoff_freq)) {
+        // zero cutoff means pass-thru
+        return;
+    }
 
     float fr = sample_freq/cutoff_freq;
     float ohm = tanf(M_PI/fr);
@@ -83,6 +87,10 @@ float LowPassFilter2p<T>::get_sample_freq(void) const {
 
 template <class T>
 T LowPassFilter2p<T>::apply(const T &sample) {
+    if (!is_positive(_params.cutoff_freq)) {
+        // zero cutoff means pass-thru
+        return sample;
+    }
     return _filter.apply(sample, _params);
 }