Avoid division by zero during estimation

pronsole.py

@@ -142,7 +142,7 @@ def estimate_duration(g):
 
                 currenttravel = hypot3d(x, y, z, lastx, lasty, lastz)
                 distance = 2* ((lastf+f) * (f-lastf) * 0.5 ) / acceleration  #2x because we have to accelerate and decelerate
-                if distance <= currenttravel:
+                if distance <= currenttravel and ( lastf + f )!=0 and f!=0:
                     moveduration = 2 * distance / ( lastf + f )
                     currenttravel -= distance
                     moveduration += currenttravel/f