tracing: fix oops in tracepoint_update_probe_range()

Change this crash:

 BUG: unable to handle kernel NULL pointer dereference at (null)
 IP: [<ffffffff8107d4de>] tracepoint_update_probe_range+0x1f/0x9b
 PGD 13d5fb067 PUD 13d688067 PMD 0
 Oops: 0000 [#1] SMP

To a more debuggable WARN_ONCE().

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1237394936.3132.1.camel@localhost.localdomain>
[ moved the check outside the lock and added a WARN_ON(). ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>