Fix kbd_buffer iteration loop in readable_events

Fix kbd_buffer iteration loop in readable_events

* src/keyboard.c (readable_events): Wrap the event pointer back to the
start of the kbd_buffer array inside the top of the loop instead of
right before checking the loop condition, since kbd_fetch_ptr and
kbd_store_ptr point past the end of the array to mean that element 0
is next. (bug#21935)