Found by Coverity, CID 11351. If we check 'if (backend)' at one line,
not clear why we do 'if (bakcend->canResume())' two lines above without any test.
Also, 'delete backend' is noop if backend is nullptr, so ...
even this 'if (backend)' is ... not needed.

Change-Id: I7a46ce04baeb9251debb7b246954911df58880ca
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>