[PATCH] perf: fix a segfault problem.

[Wang Nan <wangnan0@huawei.com>]

Without this patch, perf report cause segfault if pass "" as '-t':

  $ perf report -t ""

    # To display the perf.data header info, please use --header/--header-only options.
    #
    # Samples: 37  of event 'syscalls:sys_enter_write'
    # Event count (approx.): 37
    #
    # Children    SelfCommand   Shared Object         Symbol
    Segmentation fault

This patch avoid the segfault by checking empty string for
'symbol_conf.field_sep'.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
---
 tools/perf/util/sort.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/sort.c b/tools/perf/util/sort.c
index 4593f36..7f563a0 100644
--- a/tools/perf/util/sort.c
+++ b/tools/perf/util/sort.c
@@ -31,7 +31,8 @@ static int repsep_snprintf(char *bf, size_t size, const char *fmt, ...)
 
 	va_start(ap, fmt);
 	n = vsnprintf(bf, size, fmt, ap);
-	if (symbol_conf.field_sep && n > 0) {
+	if (symbol_conf.field_sep && n > 0 &&
+			(symbol_conf.field_sep[0] != '\0')) {
 		char *sep = bf;
 
 		while (1) {
-- 
1.8.3.4