curl_json plugin: avoid accessing off the end of the avl_tree_s

curl_json plugin: avoid accessing off the end of the avl_tree_s

It's not written this way, but really we have a union { *key; *tree; }
which is differentiated by checking for the presence a magic field
which only exists in key.  This leads to accesses off the end of the
tree.  Putting the magic at start of the key avoids this.

Signed-off-by: Florian Forster <octo@collectd.org>