Backup: clean all non printable caracters in backend path.

Previously, only non-ascii chars, shell special chars and spaces was cleaned.
Actually, some non-printable characters are ASCII and they must be cleaned too.

Change-Id: Iff91035003a448bf75307354c4011c53b8e19f10
Fix possible crash in db_exec_sql.

Change-Id: I4d8ec749e45de1f5b52294f8d8f1c9ef3b60b7b8