tests/9p: fix potential leak in v9fs_rreaddir()

Free allocated directory entries in v9fs_rreaddir() if argument
`entries` was passed as NULL, to avoid a memory leak. It is
explicitly allowed by design for `entries` to be NULL. [1]

[1] https://lore.kernel.org/all/1690923.g4PEXVpXuU@silver

Reported-by: Coverity (CID 1487558)
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <E1psh5T-0002XN-1C@lizzy.crudebyte.com>

tests/qtest/libqos/virtio-9p-client.c

@@ -594,6 +594,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
 {
     uint32_t local_count;
     struct V9fsDirent *e = NULL;
+    /* only used to avoid a leak if entries was NULL */
+    struct V9fsDirent *unused_entries = NULL;
     uint16_t slen;
     uint32_t n = 0;
 
@@ -612,6 +614,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
             e = g_new(struct V9fsDirent, 1);
             if (entries) {
                 *entries = e;
+            } else {
+                unused_entries = e;
             }
         } else {
             e = e->next = g_new(struct V9fsDirent, 1);
@@ -628,6 +632,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
         *nentries = n;
     }
 
+    v9fs_free_dirents(unused_entries);
     v9fs_req_free(req);
 }