Folly parseJson doesn't handle minInt properly

Folly parseJson doesn't handle minInt properly

Summary:
Right now in ##parseNumber## in ##folly/json.cpp##, when a negative number is provided, both the minus sign and the digits are stored in the variable ##integral##: https://fburl.com/362938516.

This causes problem when the exact min int is provided (-9223372036854775808). Because now ##integral.size()## equals 20 (including the minus sign), which is greater than ##maxIntLen## (which is 19). We need to handle negatives separately to get the correct result.

Reviewed By: yfeldblum

Differential Revision: D3479054

fbshipit-source-id: 15c782962a5f5ee845a2a18f2145c7695ec2d546