Make QContiguousCache with zero capacity not crash

Make QContiguousCache with zero capacity not crash

These containers don't make sense and will just result in no action
being taken (all items added will simply be discarded), but it
shouldn't crash due to a division by zero.

Update the documentation to explain the fact.

Task-number: QTBUG-27339
Change-Id: Ib9acf5c0a9a826e6853e7beaf5e56511fde98dc6
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Ian Walters <ian@walters.id.au>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>