Avoid crash in samplebuilder.PopWithTimestamp

[jech]

If sample is null, PopWithTimestamp will crash with SIGSEGV.

goroutine 259 [running]:
github.com/pion/webrtc/v3/pkg/media/samplebuilder.(*SampleBuilder).PopWithTimestamp(...)
        /home/jch/go/pkg/mod/github.com/pion/webrtc/v3@v3.0.29/pkg/media/samplebuilder/samplebuilder.go:285
github.com/jech/galene/diskwriter.(*diskTrack).Write(0xc0003c8000, 0xc00041cc00, 0x35f, 0x5e0, 0x0, 0x0, 0x0)
        /home/jch/go/src/github.com/jech/galene/diskwriter/diskwriter.go:477 +0x27f
github.com/jech/galene/rtpconn.sendKeyframe(0xc0000900c0, 0x3, 0x3, 0xa413d8, 0xc0003c8000, 0xc0000f6000)
        /home/jch/go/src/github.com/jech/galene/rtpconn/rtpwriter.go:218 +0xe9
created by github.com/jech/galene/rtpconn.rtpWriterLoop
        /home/jch/go/src/github.com/jech/galene/rtpconn/rtpwriter.go:263 +0x69b
exit status 2

[codecov]

Codecov Report

Merging #1838 (6d2e932) into master (41243cd) will decrease coverage by 0.25%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1838      +/-   ##
==========================================
- Coverage   75.57%   75.31%   -0.26%     
==========================================
  Files          84       84              
  Lines        5998     6000       +2     
==========================================
- Hits         4533     4519      -14     
- Misses       1080     1092      +12     
- Partials      385      389       +4     

Flag	Coverage Δ
go	76.99% <0.00%> (-0.29%)	⬇️
wasm	71.11% <0.00%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/media/samplebuilder/samplebuilder.go	91.17% <0.00%> (-1.37%)	⬇️
internal/mux/mux.go	82.53% <0.00%> (-7.94%)	⬇️
sctptransport.go	76.11% <0.00%> (-3.34%)	⬇️
dtlstransport.go	63.88% <0.00%> (-1.86%)	⬇️
datachannel.go	82.98% <0.00%> (ø)
peerconnection.go	75.48% <0.00%> (+0.08%)	⬆️

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[Sean-Der]

Merged with 11f6a65

I just added a test, thanks for fixing this @jech !