A large invalid value could cause undefined behavior when left-shifted
by 8 later in the function.

This could occur due to integer overflow when reading the channel count from
the extradata.

Only mono 8kHz is supported.

Only mono 8kHz is supported.

For decoding it does not really matter what the sample rate is.

Although the libFLAC decoder cannot handle such a change, it is allowed by the
spec and could potentially occur with live streams.

Also, return an error on allocation failure.

Only mono 16kHz is supported.

Only mono 8kHz is supported.

Ensures the decoder did not set channel count to an insanely high value
during initialization, which could cause large memory usage when it tries to
get a buffer during decoding.