[lmi] std::vector::at() in group-quote code

[Greg Chicares]

Vadim--I notice that you've used std::vector::at() throughout the
group-quote code in preference to operator[](). Is that because

(1) it provides extra safety at an incremental cost that is negligible
in the context of generating PDF files, or

(2) it actually costs nothing (when it doesn't throw) with a modern
compiler such as we're using, even on msw with sjlj semantics?

I'm assuming that the reason is (1). But if, as I can hardly imagine,
(2) also holds, then perhaps I should use at() globally.