input: Write a python function to count the frequency of characters in a string and return the updated dictionary.
  assert freq_char_dict({'a': 0, 'b': 0, 'c': 0}, 'abacab') == {'a': 3, 'b': 2, 'c': 1}
  assert freq_char_dict({'a': 0, 'b': 0, 'c': 0}, 'cccbbb') == {'a': 0, 'b': 3, 'c': 3}
  assert freq_char_dict({'x': 0, 'y': 0, 'z': 0}, 'xyzzyx') == {'x': 2, 'y': 2, 'z': 2}
output: def freq_char_dict(char_dict, string):
      for char in string:
          if char in char_dict:
              char_dict[char] += 1
      return char_dict