ShellNetworkAgent: don't access request fields if the operation is cancelled

ShellNetworkAgent: don't access request fields if the operation is cancelled

When the operation is cancelled by NetworkManager, the request is
cancelled immediately. Later when gnome-keyring invokes the callback
notifying the error we must therefore not access its memory.
Previously the callback would mistakenly treat "cancelled" (which
indicates a programmatic cancel) as "denied" (which means the user
clicked "Cancel" on the keyring prompt)

https://bugzilla.gnome.org/show_bug.cgi?id=658484