Fix crashes due to dangling external decoder pointer.
Fix crashes due to dangling external decoder pointer.

When checking whether we need to release external decoder,
we have to do pointer comparison. We can't rely on payload
types, because payload types can be stale (e.g. before we
decode the first video frame after RegisterReceiveCodec).
This leaves a dangling pointer to external decoder, which
leads to crashes later, after we actually delete the
external decoder object.
This change has been verified in Chromecast code tree.

BUG=chromium:335539
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12049004

Patch from Sergey Volk <servolk@chromium.org>.

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@5922 4adac7df-926f-26a2-2b94-8c16560cd09d

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