tracing: Fix failure path in ftrace_graph_write()

Don't call trace_parser_put() on uninitialized trace_parser.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4AB86639.3000003@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>