DRTVWR-575: Fix possible bad indexing in LLSD::operator[](size_t).

One could argue that passing a negative index to an LLSD array should do
something other than shrug and reference element [0], but as that's legacy
behavior, it seems all too likely that the viewer sometimes relies on it.

This specific problem arises if the index passed to operator[]() is negative
-- either with the previous Integer parameter or with size_t (which of course
reinterprets the negative index as hugely positive). The non-const
ImplArray::ref() overload checks parameter 'i' and, if it appears negative,
sets internal 'index' to 0.

But in the next stanza, if (index >= existing size()), it calls resize() to
scale the internal array up to one more than the requested index. The trouble
is that it passed resize(i + 1), not the adjusted resize(index + 1).

With a requested index of exactly -1, that would pass resize(0), which would
result in the ensuing array[0] reference being invalid.

With a requested index less than -1, that would pass resize(hugely positive)
-- since, whether operator[]() accepts signed LLSD::Integer or size_t,
resize() accepts std::vector::size_type. Given that the footprint of an LLSD
array element is at least a pointer, the number of bytes required for
resize(hugely positive) is likely to exceed available heap storage.

Passing the adjusted resize(index + 1) should defend against that case.