Fix for replacement of values with mask, when phi_ex is zero.
This can be spin specific. Systemtest: Relax_disp.test_hansen_cpmg_data_to_lm63 starts to fail:
Task #7807 (https://gna.org/task/index.php?7807): Speed-up of dispersion models for Clustered analysis.
Authored by: tlinnet 2014-06-13
Parent: [r23931]
Child: [r23933]
