rational: assert the denominator is nonzero on reduce

this placates clang-analyzer

rational.cpp

@@ -17,6 +17,7 @@
 #include "rational.hpp"
 
 #include "maths.hpp"
+#include "debug.hpp"
 
 #include <cstdint>
 
@@ -109,6 +110,7 @@ template <typename T>
 rational<T>
 rational<T>::reduced (void) const
 {
+    CHECK_NEZ (d);
     auto x = std::gcd (abs (n), abs (d));
 
     return { n / x, d / x };