fix mistaken variable used to infer number of masks

include/evalhyd/detail/determinist/evaluator.hpp

@@ -166,7 +166,7 @@ namespace evalhyd
                 // determine size for recurring dimensions
                 n_srs = q_prd.shape(0);
                 n_tim = q_prd.shape(1);
-                n_msk = msk.shape(1);
+                n_msk = t_msk.shape(1);
                 n_exp = b_exp.size();
 
                 // drop time steps where observations or predictions are NaN