Debugger: Fix "Restart debugging session" regression

Introduced with the support for fork-follows-child.
With 'Restart', we temporarily don't have a living
thread, so that can't be taken as indication for
ending the debugging session.

Task-number: QTCREATORBUG-14393
Change-Id: I0070b9ca7e6897dab48bdbcf493d03df4709be98
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>