write: Make get_btr_value() return void.

write: Make get_btr_value() return void.

Asking upper buffer tree nodes for the sample rate and the channels
count should never fail, because the writer only asks if there is
already some output data to process.

So, replace the tests by assertions, change the return value
of get_btr_value() to void and fix up all callers accordingly.
This simplifies the code a bit.