Continue using CBT data if there is not enough bitmap data
Continue using CBT data if there is not enough bitmap data

(cherry picked from commit 19b9f1ee1869ff2eef4c18e19bcd402408c9858f)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend