Fix #21000 - Deleting the last annotation of a segment crashes

In Segment::removeAnnotation(), deleting the last annotation fools the "i != _annotations.end()" look test.
Fixed by adding a break after the deletion.
Merge pull request #336 from mgavioli/Fix_21000_Crash_on_deleting_segm_annotation

Fix #21000 - Deleting the last annotation of a segment causes crash