[flashdisplay] Revert insertLegacyChildAt to original state

Was effectively reverted already in 2c675fa3. This is only to simplify
the code.
[variable] currentCallContext is NULL when inserting legacy children

Fixes a segfault in bug 1098107
[DefineSoundTag] Sound tag constructor was being called twice
[NetStream] Disable exceptions on FFMpegStreamDecoder stream

Let the decoder handle EOF and other conditions by returning 0 in the
read function.

Exceptions cause problems according to bug 818609.

Another problem I was seeing was that an EOF exception was thrown
while the decoder was looking for metadata on a short video, and the
expected AS events were never dispatched.
[FFMpegStreamDecoder] Fix a segfault when the video has no audio stream
[NetStream] Support videos without audio