client: Fix integer wrap around in ring_peek()

client: Fix integer wrap around in ring_peek()

Peeking beyond either end of the ring buffer must return EOF.  We
first compute the index, then check whether it's in range.

Unfortunately, the index computation r->prod - -n can wrap around
while r->prod is still <= RING_SIZE.  If it happens, ring_peek()
returns r->buf[(r->prod - -n) % RING_SIZE] instead of EOF.

Currently harmless, because no caller peeks out of range.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>