Remove implicit downcast in producer_fec_fuzzer.cc.
Remove implicit downcast in producer_fec_fuzzer.cc.

Speculative fix for DrFuzz.

BUG=
TBR=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1664453002 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#11465}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2ab815779c7c8dd1d9b4378da3e834e6f80f0889

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk