lttng lib: ring buffer move null pointer check to open

lttng lib: ring buffer move null pointer check to open

Let's move the NULL buf check to the file "open", where it belongs.  The
"open" file operation is the actual interface between lib ring buffer
and the modules using it.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>