Clear old decoders after recreating the receiver.
Clear old decoders after recreating the receiver.

Prevents UAF when switching decoder capabilities and the
previously-supported decoder is currently being received on.

BUG=chromium:565967
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1490233010 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10898}

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