Adjust buffer text indirection counters at the end of Fkill_buffer.

Adjust buffer text indirection counters at the end of Fkill_buffer.
* buffer.c (Fkill_buffer): Adjust indirection counters when the
buffer is definitely dead.  This should really fix an issue reported
by Christoph Scholtes again.  (Bug#12007).
(init_buffer_once): Initialize indirection counters of
buffer_defaults and buffer_local_symbols (for sanity and safety).