The AnnotationRulerColumn.doPaint1() does not check for an error return
value of -1 from either getInclusiveTopIndexStartOffset() or
getExclusiveBottomIndexEndOffset(). This caused an assertion failure
when retrieving an annotation iterator with a negative length. This
change adds checks for the values returned from either method.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1776