Fix Timestamp Unmarshaling for pre-epoch times

The `%` operator is the remainder, so was resulting in negative
nano components. The UnixNano call can also overflow, notably for
the zero time. Switch to using `Unix()` directly for seconds, and
`Nanosecond()` for the nanosecond component.

jsonpb/jsonpb.go

diff --git a/jsonpb/jsonpb.go b/jsonpb/jsonpb.go
index 38f86c5..1fc8ae8 100644
--- a/jsonpb/jsonpb.go
+++ b/jsonpb/jsonpb.go
@@ -608,11 +608,8 @@
 			if err != nil {
 				return fmt.Errorf("bad Timestamp: %v", err)
 			}
-			ns := t.UnixNano()
-			s := ns / 1e9
-			ns %= 1e9
-			target.Field(0).SetInt(s)
-			target.Field(1).SetInt(ns)
+			target.Field(0).SetInt(int64(t.Unix()))
+			target.Field(1).SetInt(int64(t.Nanosecond()))
 			return nil
 		}
 	}

jsonpb/jsonpb_test.go

diff --git a/jsonpb/jsonpb_test.go b/jsonpb/jsonpb_test.go
index d2f6604..bd1449c 100644
--- a/jsonpb/jsonpb_test.go
+++ b/jsonpb/jsonpb_test.go
@@ -468,6 +468,8 @@
 
 	{"Duration", Unmarshaler{}, `{"dur":"3.000s"}`, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 3}}},
 	{"Timestamp", Unmarshaler{}, `{"ts":"2014-05-13T16:53:20.021Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: 14e8, Nanos: 21e6}}},
+	{"PreEpochTimestamp", Unmarshaler{}, `{"ts":"1969-12-31T23:59:58.999999995Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: -2, Nanos: 999999995}}},
+	{"ZeroTimeTimestamp", Unmarshaler{}, `{"ts":"0001-01-01T00:00:00Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: -62135596800, Nanos: 0}}},
 
 	{"DoubleValue", Unmarshaler{}, `{"dbl":1.2}`, &pb.KnownTypes{Dbl: &wpb.DoubleValue{Value: 1.2}}},
 	{"FloatValue", Unmarshaler{}, `{"flt":1.2}`, &pb.KnownTypes{Flt: &wpb.FloatValue{Value: 1.2}}},