tests: Fix logic to not have possible NULL deref

tests: Fix logic to not have possible NULL deref

It's possible that virBitmapNewString returns NULL with an error
string (and not an allocation failure that would abort); however, if
virBitmapToString is called with a NULL @bitmap, then it will fail
in an ugly manner. So rather than have if (!map && !str) logic, split
the checks for each variable.

Found by Coverity

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>