Fix double free and out of order delete session

Fix double free and out of order delete session

Failing to create metadata, the data structure was freed but the pointer
was not set to NULL. When deleting the metadata of that session (that
has failed), the pointer was not NULL creating a "valid" pointer for the
check and creating a double free error. This failure happens when the
session daemon reached the maximum number of open files.

Also, on destroy trace, the ust app session was freed and deref. just
after creating an invalid pointer access.

Finally, set the started flag of a ust app session to 0 when stopping
the session.

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