SIGFPE: Division by zero

[derselbst]

When constructing the BoxFilters here:

basics/include/signalsmith-basics/limiter.h

Line 70 in 012d2be

signalsmith::envelopes::BoxFilter<Sample> smoother1{0}, smoother2{0};

The BoxFilter's constructor will be called with maxLength==0:

https://github.com/Signalsmith-Audio/dsp/blob/2d20161915e733f117545c6be8cd3275a739a1e3/envelopes.h#L187

This causes a division by zero a few lines below, crashing any program that traps on SIGFPE:

https://github.com/Signalsmith-Audio/dsp/blob/2d20161915e733f117545c6be8cd3275a739a1e3/envelopes.h#L199