Impact: fix kernel crash that can trigger during tracing

If we try to remove a probe that has not been already registered,
the tracepoint_entry_remove_probe() function will dereference a NULL
pointer.

Check the probe before removing it to avoid crashes.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Acked-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>