input: Write a function to debug the given Python code where function f1 does not return a value, and function f2 relies on f1 to return the incremented value of x. Also, provide the corrected code with assert test cases.
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  assert f2(2) == 9
  assert f2(0) == 1
  assert f2(-3) == 4
  assert f2(10) == 121
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)