message: Resurrect quat slerp NaN protection.
  In my previous NaN-related commit, I added a conditional similar to the
  one seen in glMatrix, but this was insufficient for the interpolation
  seen in a glTF skinning test because a divide-by-zero was occuring later
  in the function. This commit simply adds back the previous protection.
diff: @@ -256,6 +256,9 @@ public:
  const T a0 = a * (1 - t);
  const T a1 = a * t;
  const T sina = sin(a);
  + if (sina < value_eps) {
  + return lerp(p, q, t);
  + }
  const T isina = 1 / sina;
  const T s0 = std::sin(a0) * isina;
  const T s1 = std::sin(a1) * isina;
  