parse-events: Let pevent_free() take a NULL pointer
The pevent_free() should ack like other free()s and allow a
NULL pointer to be passed to it which makes error handling a bit
easier for the users of pevent_free().
Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Arnaldo Carvalho de Melo <acme@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Arun Sharma <asharma@fb.com> Cc: Namhyung Kim <namhyung.kim@lge.com> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>