mp3dec: Avoid possible endless loop.

mp3dec: Avoid possible endless loop.

If an error occurs during frame decode at the end of an 8K buffer,
the code in mp3dec.c might loop forever because we miss to consume
the data of all frames that have been decoded so far during this
iteration of the scheduler loop.

The two callers of used_mad_buffer_bytes() both call btr_consume()
next, and this fix requires to call the same two function once more
from another location. So it is natural to move the btr_consume()
call into used_mad_buffer_bytes() and rename the latter function
to mp3dec_consume().