Fix checkNegotiationNeeded panic after ReplaceTrack(nil)

[aalekseevx]

Description

I have encountered the following application panic with peerConnection using OnNegotiationNeeded handler:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x1677cea]

goroutine 1346 [running]:
github.com/pion/webrtc/v3.(*PeerConnection).checkNegotiationNeeded(0xc0006bb680)
        /-S/vendor/github.com/pion/webrtc/v3/peerconnection.go:403 +0x16ca
github.com/pion/webrtc/v3.(*PeerConnection).negotiationNeededOp(0xc0006bb680)
        /-S/vendor/github.com/pion/webrtc/v3/peerconnection.go:342 +0x10e
github.com/pion/webrtc/v3.(*operations).start(0xc0017fb398)
        /-S/vendor/github.com/pion/webrtc/v3/operations.go:93 +0x63
created by github.com/pion/webrtc/v3.(*operations).Enqueue
        /-S/vendor/github.com/pion/webrtc/v3/operations.go:41 +0x1e5

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
peerconnection.go	100.00%

📢 Thoughts on this report? Let us know!.

[Sean-Der]

LGTM @aalekseevx mind fixing commit message, then we can merge!

[aalekseevx]

@Sean-Der, updated!

[aalekseevx]

@Sean-Der, I believe this patch is worth porting to v3, what do you think?