fix: issue when airfoil defined but not used (closes #7)

src/classes/@ElemPerf/getclcd.m

@@ -59,7 +59,9 @@ function [cl, cd] = getclcd(self, aoaVect, reyVect, i)
 
         for i = 1:length(Af)
             idx = (iAf == i);
-            [cl(idx), cd(idx)] = Af(i).Polar.getcoeffs(aoaVect(idx), reyVect(idx));
+            if idx ~= 0
+                [cl(idx), cd(idx)] = Af(i).Polar.getcoeffs(aoaVect(idx), reyVect(idx));
+            end
         end
 
     else