calc: Set maximum digit length to 9

[Bennctu]

According to #25, we find that the input number is random if the digits of the number is larger than 9 digits. The type of calc->stack[0] is int, and the maximum value for an int is 2,147,483,647. Conservatively, we ensure that when the length reaches 9 digits, pressing the digit button will not increase the number of digits.

[jserv]

Thank @Bennctu for contributing. I amended the git commit message. See https://cbea.ms/git-commit/ for improving.