Because of the data types in use here, d_ninput_items_required can overflow and become negative, which then causes bad addressing of the input buffer in a block's work function. Even if we were using unsigned types, we might still have situations of overflow; it wouldn't crash in this case but cause wrong addressing, anyways. We might be able to catch and handle this situation better. This fix tests for the condition and shuts down the flowgraph with an error message. Basically, parameters that cause this effect are well out of bounds of what we want to handle, anyways.