fix timestamp encode error after some hours (#206)

previous formula was: uint32(a + uint32(b))
this formula overflows two times, while it should overflow once.
new formula is: uint32(uint64(a) + uint64(b))

pkg/rtptime/encoder_test.go

@@ -0,0 +1,14 @@
+package rtptime
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestEncoder(t *testing.T) {
+	e := NewEncoder(90000, 12345)
+
+	ts := e.Encode(0)
+	require.Equal(t, uint32(12345), ts)
+}