ParseSourceDateEpoch used strconv.ParseInt, which accepts a leading sign, so a
negative SOURCE_DATE_EPOCH was silently accepted and produced a pre-1970
timestamp with a nil error. That contradicts the function's own doc ("returns
an error if ... not well-formatted") and the reproducible-builds spec, where
SOURCE_DATE_EPOCH is a non-negative number of seconds since the Unix epoch.
Reject negative values and add a regression test.
Signed-off-by: Nikolaus Schuetz <nikolauspschuetz@gmail.com>
41f6f0e877