remove warning when decoding VP8 or VP9 (#2159)

avoid printing 'received a non-starting fragment without any previous starting fragment'

internal/formatprocessor/vp8.go

@@ -81,7 +81,7 @@ func (t *formatProcessorVP8) Process(unit Unit, hasNonRTSPReaders bool) error {
 
 			frame, pts, err := t.decoder.Decode(pkt)
 			if err != nil {
-				if err == rtpvp8.ErrMorePacketsNeeded {
+				if err == rtpvp8.ErrNonStartingPacketAndNoPrevious || err == rtpvp8.ErrMorePacketsNeeded {
 					return nil
 				}
 				return err

internal/formatprocessor/vp9.go

@@ -81,7 +81,7 @@ func (t *formatProcessorVP9) Process(unit Unit, hasNonRTSPReaders bool) error {
 
 			frame, pts, err := t.decoder.Decode(pkt)
 			if err != nil {
-				if err == rtpvp9.ErrMorePacketsNeeded {
+				if err == rtpvp9.ErrNonStartingPacketAndNoPrevious || err == rtpvp9.ErrMorePacketsNeeded {
 					return nil
 				}
 				return err