Properly handle LONG_MIN in timedata.cpp

Summary: Because there is one more negative value possible than positive value, this code does not handle LONG_MIN properly. The obvious solution is to consider the absolute value unsigned (duh!).

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6020