Modify Strip Silence so that it can't create regions with -ve lengths

Modify Strip Silence so that it can't create regions with -ve lengths

AFAICT this could happen if a region's end time (on the timeline) was earlier than the end time in the actual recording. This could cause a situation where the last block of detected silence would have an end time greater than the end tome for the region being processed. Strip Silence would create its new regions - but the last one it created would usually come out with a negative duration.