tracepoints: Prevent null probe from being added

Somehow tracepoint_entry_add_probe() function allows a null probe function.
And, this may lead to unexpected results since the number of probe
functions in an entry can be counted by checking whether a probe is null
or not in the for-loop.
This patch prevents a null probe from being added.
In tracepoint_entry_remove_probe() function, checking probe parameter
within the for-loop is moved out for code efficiency, leaving the null probe
feature which removes all probe functions in the entry.

Link: http://lkml.kernel.org/r/1365991995-19445-1-git-send-email-kpark3469@gmail.com



Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Sahara <keun-o.park@windriver.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>