It was possible to cause a JS error when creating a ranged comment that
started at the very end of the first line (selecting no content on that
line). The relevant null-guard needed an additional set of parens to
avoid evaluating the second OR operand with a bad argument in this case.
Addresses the null-guard boolean expressions in `_normalizeStart` and
`_normalizeEnd` and reduces the number of calls to `_getLength` from
thrice to once per iteration. Adds a relevant unit test.
Change-Id: I98848f9f6089fd3240bda175765770c9f9c5ba30
1678d988ae