NetworkAgent: Fix double-unref in get_secrets_keyring_cb()

In get_secrets_keyring_cb, we own a ref on the 'attributes' hash table
from secret_item_get_attributes), and a ref on the 'secret' object (from
secret_item_get_secret(), but in the SHELL_KEYRING_SK_TAG case, we unref
these once before breaking out of the loop, and the second time after
breaking out of the loop.

https://bugzilla.gnome.org/show_bug.cgi?id=759708