Tweak handling of reading null as raw bytes.

* NULL maps to nil not empty slice
* Handle NULL in Scan not ReadBytes

query.go

@@ -227,7 +227,11 @@ func (rows *Rows) Scan(dest ...interface{}) (err error) {
 			if vr.Type().DataType == ByteaOid {
 				*d = decodeBytea(vr)
 			} else {
-				*d = vr.ReadBytes(vr.Len())
+				if vr.Len() != -1 {
+					*d = vr.ReadBytes(vr.Len())
+				} else {
+					*d = nil
+				}
 			}
 		case *int64:
 			*d = decodeInt8(vr)

query_test.go

@@ -817,8 +817,8 @@ func TestReadingNullByteArray(t *testing.T) {
 		t.Fatalf("conn.QueryRow failed: ", err)
 	}
 
-	if len(a) != 0 {
-		t.Errorf("Expected 'a' to have length 0, but it was: ", len(a))
+	if a != nil {
+		t.Errorf("Expected 'a' to be nil, but it was: %v", a)
 	}
 }
 
@@ -838,8 +838,8 @@ func TestReadingNullByteArrays(t *testing.T) {
 		if err := rows.Scan(&a); err != nil {
 			t.Fatalf("failed to scan row", err)
 		}
-		if len(a) != 0 {
-			t.Errorf("Expected 'a' to have length 0, but it was: ", len(a))
+		if a != nil {
+			t.Errorf("Expected 'a' to be nil, but it was: %v", a)
 		}
 	}
 	if count != 2 {

value_reader.go

@@ -113,7 +113,8 @@ func (r *ValueReader) ReadBytes(count int32) []byte {
 		return nil
 	}
 
-	if count == -1 {
+	if count < 0 {
+		r.Fatal(errors.New("count must not be negative"))
 		return nil
 	}