minor updates to ffmpeg video-editor and silence detection based on gregs review of the feature in 1.6.3

Steps to reproduce

Greg found some issues (possible NPE's, and similar issues) in PR #644 that are now fixed for 1.6.3, but the patches also useful for 2.x branches.

Fixed and reviewed
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Assignee

Rüdiger Rolf

Reporter

Rüdiger Rolf

Severity

Crash/Hang