#69 Fix crash when QwtSetSample does not contain samples

[Povilas Kanapickas]

The attached patch fixes a crash when drawing QwtSetSample without sub-samples. This use case is possible in complex histogram graps where each bar corresponds to a collection of entities, not some physical value. Thus it's possible that some samples contain nothing.

[Uwe Rathmann]

The patch returns an invalid rectangle in both directions, while we have a proper value for at least one direction. So I didn't take the patch as it is, but the crash should be solved in all branches >= 6.1