ct_clamp: Check sample() return value is not NaN (#921)

Don't try to update CT clamp's state with NaN values returned from the
underlaying sensor.  A single IO error in the sensor code will cause a
NaN to be returned and if we use that in CTClampSensor's floating point
maths both sample_sum_ and offset_ will become NaN and from there every
future calculation will use the NaN offset_ and return NaN too.

esphome/components/ct_clamp/ct_clamp_sensor.cpp

@@ -64,6 +64,8 @@ void CTClampSensor::loop() {
 
   // Perform a single sample
   float value = this->source_->sample();
+  if (isnan(value))
+    return;
 
   if (this->is_calibrating_offset_) {
     this->sample_sum_ += value;