Fix: deny overwrite mode and num subbuf less than 2

Also fixed in UST and modules with commit:

(modules) 5140d2b3070f211e6d8a0ad7cb0a190e8b6f3e28
(ust) 3d8e9399b2fb9000b3b55e488a7d04184b5bb56e

Fixes #793

Signed-off-by: David Goulet <dgoulet@efficios.com>