For this there are two fixes:

Define the walls as sets of points (a bent line) and the balls movement as a vector. Then just test the vector against the line(s). This requires a good bit of algebra knowledge, but can be a better system in the long run for collisions (just find the gradient of the line) and mass hitTesting.

OR

Do a set of checks, ie. one for every 10px the ball will travel that frame.