Made a change to astronomy.cs template that didn't get checked in.

source/csharp/astronomy.cs

@@ -3263,7 +3263,7 @@ namespace CosineKitty
         {
             int index = (int) Math.Floor(frac);
             if (index < 0)
-                return index;
+                return 0;
             if (index >= nsteps)
                 return nsteps-1;
             return index;