Remove packet initializer in RtpRtcpRtxNackTest.
Remove packet initializer in RtpRtcpRtxNackTest.

Fixes RtpRtcpRtxNackTest to not use uninitialized data when not sending
RTX.

BUG=webrtc:3183
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1427653007

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10665}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 740c4f11e0d3b409c5444b328859754d2a717e33

jchavanton/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,jchavanton/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk