Bugfix in packet_length calculation

One of the bitshifts used to construct tmplen (lines 230-231) was inconsistent with the other expressions:

(input[mypos2++].charCodeAt() << 8)

causing following error:

TypeError: Cannot call method 'charCodeAt' of undefined
    at Object.module.exports.read (/home/scott/dev/keystone/fetch/node_modules/openpgp/src/packet/packet.js:231:16)

Corrected to 

(input.charCodeAt(mypos2++) << 8)
Merge pull request #245 from inovari/master

Bugfix in packet_length calculation