client: Fix settings crash on missing net identity

client: Fix settings crash on missing net identity

When fetching the client identity in Network settings, make sure the
identity exists before trying to convert it into a CertIdentity.

This resolves a crash if an identity is deleted improperly or if no
identities are specified.

Test case:
1.  Clear the configuration for Quassel monolithic
2.  Start Quassel monolithic
3.  Cancel the setup wizard (before creating any identity)
4.  Go to Settings -> Configure Quassel... (F7)
5.  Navigate to IRC -> Identities, confirm that no identity exists
6.  Navigate to IRC -> Networks
7.  Add a new network (details don't matter)
8.  Observe results

NOTE: This fix was contributed by "kater" on the Quassel IRC bug
tracker.  I'm not sure of a better way to provide attribution, as I
don't know if they want their email address part of Quassel's public
record.

See the original patch here:
https://bugs.quassel-irc.org/issues/1409

Fixes #1409