[PATCH] selftest: net: Fix error message if empty variable

[Alessandro Zanni]

Fix to avoid cases where the `res` shell variable is
empty in script comparisons.

The issue can be reproduced with the command:
make kselftest TARGETS=net

It solves the error:
./tfo_passive.sh: line 98: [: -eq: unary operator expected

Signed-off-by: Alessandro Zanni <alessandro.zanni87@xxxxxxxxx>
---
 tools/testing/selftests/net/tfo_passive.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/tfo_passive.sh b/tools/testing/selftests/net/tfo_passive.sh
index 80bf11fdc046..2655931b2396 100755
--- a/tools/testing/selftests/net/tfo_passive.sh
+++ b/tools/testing/selftests/net/tfo_passive.sh
@@ -95,7 +95,7 @@ wait
 res=$(cat $out_file)
 rm $out_file
 
-if [ $res -eq 0 ]; then
+if [ -n "$res" ] && [ $res -eq 0 ]; then
 	echo "got invalid NAPI ID from passive TFO socket"
 	cleanup_ns
 	exit 1
-- 
2.43.0