tracing/filters: fix off-by-one bug

We should leave the last slot for the ending '\0'.

[ Impact: fix possible crash when the length of an operand is 128 ]
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4A0CDC8C.30602@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events_filter.c

@@ -736,7 +736,7 @@ static inline void clear_operand_string(struct filter_parse_state *ps)
 
 static inline int append_operand_char(struct filter_parse_state *ps, char c)
 {
-	if (ps->operand.tail == MAX_FILTER_STR_VAL)
+	if (ps->operand.tail == MAX_FILTER_STR_VAL - 1)
 		return -EINVAL;
 
 	ps->operand.string[ps->operand.tail++] = c;