SampleBuilder should Unmarshal after collecting

Before we incorrectly ran Unmarshal on every RTP Payload. This worked
for Opus and VP8 because it didn't care about the Payload Headers.
However this would break H264.

Relates to #1652

pkg/media/samplebuilder/samplebuilder.go

@@ -81,7 +81,7 @@ const secondToNanoseconds = 1000000000
 // We have a valid collection of RTP Packets
 // walk forwards building a sample if everything looks good clear and update buffer+values
 func (s *SampleBuilder) buildSample(firstBuffer uint16) (*media.Sample, uint32) {
-	data := []byte{}
+	payloadBuffer := []byte{}
 
 	for i := firstBuffer; s.buffer[i] != nil; i++ {
 		if s.buffer[i].Timestamp != s.buffer[firstBuffer].Timestamp {
@@ -104,15 +104,15 @@ func (s *SampleBuilder) buildSample(firstBuffer uint16) (*media.Sample, uint32)
 				s.releasePacket(j)
 			}
 
-			return &media.Sample{Data: data, Duration: time.Duration((float64(samples)/float64(s.sampleRate))*secondToNanoseconds) * time.Nanosecond}, s.lastPopTimestamp
-		}
+			data, err := s.depacketizer.Unmarshal(payloadBuffer)
+			if err != nil {
+				return nil, 0
+			}
 
-		p, err := s.depacketizer.Unmarshal(s.buffer[i].Payload)
-		if err != nil {
-			return nil, 0
+			return &media.Sample{Data: data, Duration: time.Duration((float64(samples)/float64(s.sampleRate))*secondToNanoseconds) * time.Nanosecond}, s.lastPopTimestamp
 		}
 
-		data = append(data, p...)
+		payloadBuffer = append(payloadBuffer, s.buffer[i].Payload...)
 	}
 	return nil, 0
 }