[wpilib, hal] DigitalGlitchFilter: Fix sim crash and clean up construction (#6937)

Fixes error when >3 are constructed- in java, m_filterAllocated[index] would be evaluated before the bounds check and throw IndexOutOfBounds, in c++ a vague assertion error would be thrown.

Makes DoAdd static in c++

Makes the error message when HAL_SetFilterSelect fails consistent with java

hal/src/main/native/sim/DigitalInternal.h

@@ -50,6 +50,7 @@ struct DigitalPort {
   int32_t centerPwm = 0;
   int32_t deadbandMinPwm = 0;
   int32_t minPwm = 0;
+  int32_t filterIndex = 0;
   std::string previousAllocation;
 };