ftmod_isdn: Avoid stack smashing buffer overflow in isdn_tones_run().

ftmod_isdn: Avoid stack smashing buffer overflow in isdn_tones_run().

The len variable can, in certain situations (large burst of incoming non-SLIN audio),
exceed the size of the on-stack frame buffer, which causes ftdm_buffer_read_loop() to
overwrite the dt_buffer pointer.

Use ftdm_min() to make sure len (after conversion to SLIN units) isn't larger
than the frame buffer size.

Also adds are couple more code comments.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>