linux: turn on -Wextra

sandbox/linux/suid/linux_util.c

Index: sandbox/linux/suid/linux_util.c
diff --git a/sandbox/linux/suid/linux_util.c b/sandbox/linux/suid/linux_util.c
index ded545bdbab51f596dd3c6f695374b2d2af6b60c..c5af0d0d1dc9672abdf324774e95013ab5f1e55e 100644
--- a/sandbox/linux/suid/linux_util.c
+++ b/sandbox/linux/suid/linux_util.c
@@ -83,8 +83,9 @@ bool FindProcessHoldingSocket(pid_t* pid_out, ino_t socket_inode) {
       continue;
 
     while ((dent = readdir(fd))) {
-      if (snprintf(buf, sizeof(buf), "/proc/%lu/fd/%s", pid_ul,
-                   dent->d_name) >= sizeof(buf) - 1) {
+      int printed = snprintf(buf, sizeof(buf), "/proc/%lu/fd/%s", pid_ul,
+                             dent->d_name);
+      if (printed < 0 || printed >= (int)(sizeof(buf) - 1)) {
         continue;
       }