9pfs: fix potential segfault during walk

If the call to fid_to_qid() returns an error, we will call v9fs_path_free()
on uninitialized paths.

It is a regression introduced by the following commit:

56f101ec 9pfs: handle walk of ".." in the root directory

Let's fix this by initializing dpath and path before calling fid_to_qid().
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NCédric Le Goater <clg@kaod.org>
[groug: updated the changelog to indicate this is regression and to provide
        the offending commit SHA1]
Signed-off-by: NGreg Kurz <groug@kaod.org>

hw/9pfs/9p.c

@@ -1333,13 +1333,14 @@ static void v9fs_walk(void *opaque)
         goto out_nofid;
     }
 
+    v9fs_path_init(&dpath);
+    v9fs_path_init(&path);
+
     err = fid_to_qid(pdu, fidp, &qid);
     if (err < 0) {
         goto out;
     }
 
-    v9fs_path_init(&dpath);
-    v9fs_path_init(&path);
     /*
      * Both dpath and path initially poin to fidp.
      * Needed to handle request with nwnames == 0