Fix out of order channel insert in hash table

The channel, ust-app side, was added to the hash table of the session
even if the creation failed on the tracer side. When enabling event for
that channel, a segfault was hit since the channel did not exist on the
tracer side and channel_data was NULL at this point.

Now the channel insertion in the hash table is done *after* enabling it
on the tracer side.

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