e2pg: exotic bugfix. dest range filter when stop < start

The prior code assumed that if r.stop was 0 then destRange is
effectively uninitialized and therefore no filtering should be done.
However, it is possible for the stop value to be zero [1] and thus a
more thorough uninitialized check should be preformed.

[1]: See commit: 90334ab

e2pg/e2pg.go

@@ -588,7 +588,7 @@ func (r *destRange) load(ctx context.Context, pg wpg.Conn, name, srcName string)
 
 func (r *destRange) filter(blks []eth.Block) []eth.Block {
 	switch {
-	case r.stop == 0:
+	case r.start == 0 && r.stop == 0:
 		return blks
 	case len(blks) == 0:
 		return blks

e2pg/e2pg_test.go

@@ -630,6 +630,12 @@ func TestDestRanges_Filter(t *testing.T) {
 			r:     destRange{start: 15, stop: 10},
 			want:  []eth.Block(nil),
 		},
+		{
+			desc:  "[0, 10] -> [5, 0]",
+			input: br(0, 10),
+			r:     destRange{start: 5, stop: 0},
+			want:  []eth.Block(nil),
+		},
 	}
 	for _, tc := range cases {
 		got := tc.r.filter(tc.input)