Invalid values for DERRF distribution leads to "float division by zero" in update

[eivindjahren]

What happened?

Invalid values for DERRF distribution leads to "index 0 is out of bounds for axis 0 with size 0" in update

What did you expect to happen?

Config with invalid distribution shows validation error message on startup.

steps to reproduce

1. change test-data/ert/poly_example/coeff_priors to

a UNIFORM 0 1
b UNIFORM 0 2
c DERRF 0.9999999999999999 0.1900000000000001 69.04934509153614 0.1900000000000001 0.1900000000000001

2. ert gui test-data/ert/poly_example/poly.er
3. run ES-MDA in gui
   Now you get an error saying index 0 is out of bounds for axis 0 with size 0.