i9e: Fix invalid key handling.

i9e: Fix invalid key handling.

If an unmapped key is pressed repeatedly, we store the key sequence in
a 32 byte buffer until there is no more space left in the buffer. Then
we terminate the process with

	para_play: interactive.c:304: i9e_post_monitor: Assertion `len < sizeof(i9ep->key_sequence) - 1' failed.

This is not a nice way to deal with invalid input, so be a bit more
graceful and discard the buffer when it is full or when there is no
further input available at the moment.