Fix: filter: check binary op nesting

Should pass nesting + 1 as parameter rather than nesting++. Worked in
when nesting was in one direction (due to side-effect of the first ++),
but not the other.

Fixes #688

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>