tracing: Fix race where eprobes can be called before the event

tracing: Fix race where eprobes can be called before the event

The flag that tells the event to call its triggers after reading the event
is set for eprobes after the eprobe is enabled. This leads to a race where
the eprobe may be triggered at the beginning of the event where the record
information is NULL. The eprobe then dereferences the NULL record causing
a NULL kernel pointer bug.

Test for a NULL record to keep this from happening.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]/
Link: https://lore.kernel.org/linux-trace-kernel/[email protected]
Cc: Linux Trace Kernel <[email protected]>
Cc: Tzvetomir Stoyanov <[email protected]>
Cc: Tom Zanussi <[email protected]>
Cc: [email protected]
Fixes: 7491e2c442781 ("tracing: Add a probe that attaches to trace events")
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Reported-by: Rafael Mendonca <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>