Added fix for incorrect VPDigitCreator logging of main event & spillover clusters

Fix error in the logging code for VPDigitCreator where the number of spillover pixels could be over counted. Leading to a unsigned int having being assigned a negative value, giving an occasional event with an overflow in the number of signal pixels recorded. See Allen#496 (comment 7608009). My thanks to @sponce and @tevans for tracking this down.

Closed as changes included in !566 (merged)