update ffmpeg (n0.8-19006-g15e7533)
ffmpeg: Specify the folders to remove. The MSVC log file was in the folder being removed and thus leading to an error when rebuilding via the VS IDE now that gccbuild.bat error levels are properly set.
Fix incorrect calls to *scanf_s functions.

The documentation is quite unclear but it seems that all buffer sizes should be given as characters count rather bytes count.

Note that although the calls were incorrect, there was probably no real effect for users.