avcodec: Assert on codec->encode2 in encode_audio2

Assert on `avctx->codec->encode2` to avoid a SEGFAULT on the subsequent
function call.

avcodec_encode_video2() uses a similar assertion.
Calling the wrong function on a stream is a serious inconsistency
which could at other places be potentially dangerous and exploitable,
it is thus safer to stop execution and not continue with such
inconsistency after returning an error.

Commit-message-extended-by commiter
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
