Fixed invalid cc number length

The credit card number may be stripped of non-digits via `$number = preg_replace('/\D/', '', $input[$field]);`, however the length of the card number is being determined by reading the string length of `$input[$field]` instead of `$number`. This leads to an undefined index error at line 1260.
Merge pull request #25 from headzoo/patch-2

Fixed invalid cc number length