block/amend: Check whether the node exists

We should check whether the user-specified node-name actually refers to
a node.  The simplest way to do that is to use bdrv_lookup_bs() instead
of bdrv_find_node() (the former wraps the latter, and produces an error
message if necessary).

Reported-by: Coverity (CID 1430268)
Fixes: ced914d0Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-Id: <20200710095037.10885-1-mreitz@redhat.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>

block/amend.c

@@ -69,8 +69,12 @@ void qmp_x_blockdev_amend(const char *job_id,
     BlockdevAmendJob *s;
     const char *fmt = BlockdevDriver_str(options->driver);
     BlockDriver *drv = bdrv_find_format(fmt);
-    BlockDriverState *bs = bdrv_find_node(node_name);
+    BlockDriverState *bs;
 
+    bs = bdrv_lookup_bs(NULL, node_name, errp);
+    if (!bs) {
+        return;
+    }
 
     if (!drv) {
         error_setg(errp, "Block driver '%s' not found or not supported", fmt);