Note find_ust_app_context must be called with RCU read lock

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: filter bytecode and string memory leak on error

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>