_q_networkSessionStateChanged - fix weird logic

a != A || a != B looks suspiciously wrong.
Found, unfortunately, only by Coverity (CIDs 152247, 152249).

Change-Id: Ia7ec810dde2a3a33ae6fc5644a4eebf279dcffe8
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>