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-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#11465}

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc