Fix potential segfault when when depsolving
Fix potential segfault when when depsolving

This can happen when hif_transaction_ensure_source() is used before a call to
hif_transaction_set_sources() although doesn't seem to be triggered by any of
the current users.

rpm-software-management/libhif,Conan-Kudo/libhif,hughsie/libhif,hughsie/libhif,edynox/libdnf,edynox/libdnf,Conan-Kudo/libhif,rpm-software-management/libdnf,cgwalters/libhif,kalev/libhif,hughsie/libhif,rpm-software-management/libdnf,juhp/libhif,kalev/libhif,edynox/libdnf,rpm-software-management/libhif,cgwalters/libhif,rpm-software-management/libhif,Conan-Kudo/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,juhp/libhif,Conan-Kudo/libhif,hughsie/libhif,cgwalters/libhif,rpm-software-management/libdnf,cgwalters/libhif,edynox/libdnf