Fix FLAC read callback for data previously read during probing.

This was not encountered before because libflac happened to
always request at least 4 bytes for the first read callback.

Fixes https://github.com/xiph/opus-tools/issues/71

Signed-off-by: Mark Harris <mark.hsj@gmail.com>
1 job for fix_flac in 1 minute and 7 seconds (queued for 5 seconds)