Fix to CollectSamErrorMetrics getIndelOperator function to check operator type

[tfenne]

Description

The getIndelOperator function in ReadBaseStratifier takes a RecordAndOffset and is intended to return either the Insertion or Deletion Cigar element found in that read's cigar at that offset, depending on whether the RecordAndOffset represents an insertion of deletion operation.

It appears to function correctly if and only if passed RecordAndOffsets that are also correct - i.e. correctly identify an insertion or deletion within a read at an accurate offset. However, when an incorrect offset is given for an insertion (as triggered by this bug in HTSJDK, it will erroneously return any cigar element with an operator that consumes read bases.

This PR adds a test to highlight that invalid input causes this problem, and ensure that in that case null is returned instead of an incorrect cigar element.

Checklist (never delete this)

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Content

• Added or modified tests to cover changes and any new functionality
• Edited the README / documentation (if applicable)
• All tests passing on github actions

Review

• Final thumbs-up from reviewer
• Rebase, squash and reword as applicable

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[yfarjoun]

should this be logged? (can it be easily logged ?)

seems like if sli isn't buggy, this should never happen...