Fix invalid memory access from rpmdb growth estimation (RhBug:766260)

Fix invalid memory access from rpmdb growth estimation (RhBug:766260)

- The dsi retrieved from rpmtsDbDSI() would become invalid whenever
  ts->dsi structure got realloced, ie anytime when disk space is
  calculated for a different fs than where the rpmdb resides. This
  is likely to be the real issue behind RhBug:766260 and also RhBug:671056.
  Just call rpmtsUpdateDSI() directly with suitable arguments for the
  rpmdb to avoid the special (re)alloc paths.
- In addition, consider the db growth for packages with no files too.