Fix rare SRTP loss decode failure

As described in https://webrtc-review.googlesource.com/c/src/+/358360
there can be a problem when the sequence number starts near the
rollover point and there is packet loss. As linked to in that issue,
libsrtp recommends having the starting sequence number be less than
2^15 to avoid that problem.

sequencer.go

@@ -4,7 +4,6 @@
 package rtp
 
 import (
-	"math"
 	"sync"
 )
 
@@ -14,11 +13,18 @@ type Sequencer interface {
 	RollOverCount() uint64
 }
 
+// maxInitialRandomSequenceNumber is the maximum value used for the initial sequence
+// number when using NewRandomSequencer().
+// This uses only half the potential sequence number space to avoid issues decrypting
+// SRTP when the sequence number starts near the rollover and there is packet loss.
+// See https://webrtc-review.googlesource.com/c/src/+/358360
+const maxInitialRandomSequenceNumber = 1<<15 - 1
+
 // NewRandomSequencer returns a new sequencer starting from a random sequence
 // number
 func NewRandomSequencer() Sequencer {
 	return &sequencer{
-		sequenceNumber: uint16(globalMathRandomGenerator.Intn(math.MaxUint16)),
+		sequenceNumber: uint16(globalMathRandomGenerator.Intn(maxInitialRandomSequenceNumber)),
 	}
 }