tracing: trace_output.c, fix false positive compiler warning

This compiler warning:

  CC      kernel/trace/trace_output.o
 kernel/trace/trace_output.c: In function ‘register_ftrace_event’:
 kernel/trace/trace_output.c:544: warning: ‘list’ may be used uninitialized in this function

Is wrong as 'list' is always initialized - but GCC (4.3.2) does not
recognize this relationship properly.

Work around the warning by initializing the variable to NULL.

[ Impact: fix false positive compiler warning ]
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_output.c

@@ -541,7 +541,7 @@ int register_ftrace_event(struct trace_event *event)
 	INIT_LIST_HEAD(&event->list);
 
 	if (!event->type) {
-		struct list_head *list;
+		struct list_head *list = NULL;
 
 		if (next_event_type > FTRACE_MAX_EVENT) {