Don’t use reference to initially allocated avio buffer

aviocontext can resize the buffer under some circumstances. Attempting to later free the original buffer address would cause a crash
Increase size of streaming buffer

streaming buffer size is also used to detect audio format, 16kB may not be sufficient to properly detect corrupted media or FLAC ones.
The buffer will be resized to 32kB by FFmpeg once the file has been successfully opened
Let FFmpeg determine how to probe for format when using SG

This makes for much more reliable format detection.