Ensure we won't leak memory even in case of error.

CID 1134286 and 1134287.
CRenderedHdmvSubtitle: Remove an unused variable.

CID 1134288.
Update LAV Filters to 0.59.1-57-80f1385 (custom build based on 0.59.1-52-0b90b46).

Important change:
 - Update libraries (ffmpeg, libspeex, Opus, opencore-amr).
 - LAV Video decoder: Fix a decoding issue with some H264 files (fixes #3793).

Also fixes #3785.
LAV Filters: Detect when ffmpeg advices to rerun configure and do so in that case.