fix encoding of RTP packets with padding

this fixes a SIGSEGV with GStreamer

client.go

@@ -22,7 +22,7 @@ import (
 	"time"
 
 	"github.com/pion/rtcp"
-	"github.com/pion/rtp"
+	"github.com/pion/rtp/v2"
 
 	"github.com/aler9/gortsplib/pkg/auth"
 	"github.com/aler9/gortsplib/pkg/base"
@@ -765,6 +765,10 @@ func (c *Client) runReader() {
 							return
 						}
 
+						// remove padding
+						pkt.Header.Padding = false
+						pkt.PaddingSize = 0
+
 						c.tracks[trackID].rtcpReceiver.ProcessPacketRTP(now, pkt)
 						c.OnPacketRTP(trackID, pkt)
 					} else {