If the child process cannot be started, return -1.

Task-number: QTCREATORBUG-6458
Change-Id: I0392d883773f3037ac29dccc4d4a3c330efde838
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>