[PATCH] tools/9pfsd: Fix build error caused by strerror_r

[Henry Wang <xin.wang2@xxxxxxx>]

Below error can be seen when doing Yocto build of the toolstack:

| io.c: In function 'p9_error':
| io.c:684:5: error: ignoring return value of 'strerror_r' declared
  with attribute 'warn_unused_result' [-Werror=unused-result]
|   684 |     strerror_r(err, ring->buffer, ring->ring_size);
|       |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| cc1: all warnings being treated as errors

Fix it by adding a return value and check it properly.

Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
---
 tools/9pfsd/io.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/tools/9pfsd/io.c b/tools/9pfsd/io.c
index adb887c7d9..163eee6ecc 100644
--- a/tools/9pfsd/io.c
+++ b/tools/9pfsd/io.c
@@ -681,11 +681,17 @@ static void p9_error(struct ring *ring, uint16_t tag, 
uint32_t err)
 {
     unsigned int erroff;
 
-    strerror_r(err, ring->buffer, ring->ring_size);
-    erroff = add_string(ring, ring->buffer, strlen(ring->buffer));
-    fill_buffer(ring, P9_CMD_ERROR, tag, "SU",
-                erroff != ~0 ? ring->str + erroff : "cannot allocate memory",
-                &err);
+    char *ret = strerror_r(err, ring->buffer, ring->ring_size);
+
+    if ( ret )
+    {
+        erroff = add_string(ring, ring->buffer, strlen(ring->buffer));
+        fill_buffer(ring, P9_CMD_ERROR, tag, "SU",
+                    erroff != ~0 ?
+                             ring->str + erroff :
+                             "cannot allocate memory",
+                    &err);
+    }
 }
 
 static void p9_version(struct ring *ring, struct p9_header *hdr)
-- 
2.34.1