Fix memory access errors when density does not depend on function value

If InterpolationDerivatives is told not to compute the derivatives
for the interpolation value, then it sometimes leaves the corresponding
matrix entries uninitialized.  That's okay: they shall not be used
anyway.

However, the code still did use them: It multiplied them by zero.
This is okay if the matrix entries are actual values, but they may
also be NaNs.  In that case, the NaNs propagate, leading to wrong
results.  Fix this by modifying the matrix products to really omit
the uninitialized values.