Well, you cannot assume that - look at the code (there are links to it) - there are several different ways in which it will go out of bounds.
What's more, even if it were the case that the worst that could happen is that the variable gets set to zero and the program terminates gracefully without doing anything, that would not amount to a justification for not checking for an error, it would merely be an observation that, fortuitously, this particular bug is relatively harmless, perhaps doing nothing worse than leaving the user unnecessarily puzzled over what went wrong.
Even the example you give here (which is not actually to be found in the code in question, and so could not possibly be a candidate for the worst thing about it) is relatively harmless compared to potentially causing undefined behavior, or (what is sometimes at least as bad) defined behavior that apparently works but which gives subtly wrong answers. And, by the way, code can lie too: identifiers can make claims about the semantics of a variable or function that are not always true. The only way to avoid that is to use meaningless identifiers.
The more you try to brush this off and argue that comments are worse than actual errors, the more obvious it becomes that your priorities are askew here.
