tools lib traceevent: Zero should not be considered "not found" in eval_flag()

Guilherme Cox found that:

 There is, however, a potential bug if there is an item with code zero
 that is not the first one in the symbol list, since eval_flag(..)
 returns 0 when it doesn't find anything.

That is, if you have the following enums:

enum {
  FOO_START = 0,
  FOO_GO    = 1,
  FOO_END   = 2
}

and then have:

  __print_symbolic(foo, FOO_GO, "go", FOO_START, "start",
		        FOO_END, "end")

If none of the enums are known to pevent, then eval_flag() will return
zero, and it will match it to the first item in the list, which would be
FOO_GO, which is not zero.

Luckily, in most cases, the first element would be zero, and the parsing
would match out of sheer luck.
Reported-by: Guilherme Cox <cox@computer.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20150324145813.0bfe95ba@gandalf.local.homeSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/event-parse.c

@@ -3615,7 +3615,7 @@ static const struct flag flags[] = {
 	{ "HRTIMER_RESTART", 1 },
 };
 
-static unsigned long long eval_flag(const char *flag)
+static long long eval_flag(const char *flag)
 {
 	int i;
 
@@ -3631,7 +3631,7 @@ static unsigned long long eval_flag(const char *flag)
 		if (strcmp(flags[i].name, flag) == 0)
 			return flags[i].value;
 
-	return 0;
+	return -1LL;
 }
 
 static void print_str_to_seq(struct trace_seq *s, const char *format,
@@ -3705,7 +3705,7 @@ static void print_str_arg(struct trace_seq *s, void *data, int size,
 	struct print_flag_sym *flag;
 	struct format_field *field;
 	struct printk_map *printk;
-	unsigned long long val, fval;
+	long long val, fval;
 	unsigned long addr;
 	char *str;
 	unsigned char *hex;
@@ -3764,11 +3764,11 @@ static void print_str_arg(struct trace_seq *s, void *data, int size,
 		print = 0;
 		for (flag = arg->flags.flags; flag; flag = flag->next) {
 			fval = eval_flag(flag->value);
-			if (!val && !fval) {
+			if (!val && fval < 0) {
 				print_str_to_seq(s, format, len_arg, flag->str);
 				break;
 			}
-			if (fval && (val & fval) == fval) {
+			if (fval > 0 && (val & fval) == fval) {
 				if (print && arg->flags.delim)
 					trace_seq_puts(s, arg->flags.delim);
 				print_str_to_seq(s, format, len_arg, flag->str);