Fixes decryption error from some clients

Fixes the issues where decryption fails due to this error:
Decrypt ERROR: 'L' format requires 0 <= number <= 4294967295

Copy paste of patch from weechat here:
https://github.com/freshprince/weechat-fish/issues/14
Merge pull request #2 from 4C3/master

Fixes decryption error from some clients