Fix bug uncovered by changing alloca to auto buffer.

Fix bug uncovered by changing alloca to auto buffer.

* coding.c (growable_destination): New function.
(produce_chars): Use it for sanity checks.  Do not fiddle with
dst_end if the source and destination are both nil, as it's
the caller's responsibility to avoid overlap.
* keyboard.c (read_decoded_event_from_main_queue):
The destination must be MAX_MULTIBYTE_LENGTH times the max source
length, not 4 times, to prevent decode_coding_c_string from trying
to reallocate a destination.  This removes the need for the FIXME.

Fixes: debbugs:18410