stdiostream: Fix unhandled ftello failures

Fixes CIDs #1109579 & #1109582.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
utils: Check if (re-)opening the codec fails

Fixes CID #1109578.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
audiosource: Check if we reopened the resample context properly

Fixes CID #1109577.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
lavfvideo: Initialize DummyPTS and DummyPos

It has no pratcical effect, but it removes the use of
uninitialized data.

Fixes CID #1109584.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>