Fix crash caused by g_network_monitor_base_add_network()
Fix crash caused by g_network_monitor_base_add_network()

This avoids a crash when starting Evolution, and fixes the
network-monitor and network-monitor-race test cases on my developer
workstation. (I assume the CI is not crashing due to lack of network
access there.)

Problem is that if a network already exists in the networks table,
g_hash_table_add() "destroys" (unrefs) it before adding the new one
(which we failed to ref before adding). This means we just accidentally
lost a ref. In practice, the network gets unexpectedly destroyed here
before returning.

Fixes #2020

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib