fix(windows): PreservedKeyMap::MapUSCharToVK line order bug

I discovered this while working on #4586. The code in question was
updating `*puKey` and then trying to dereference the array on the basis
of its new value, which (a) would give the wrong result, and (b) could
be reading off the end of the array (although not crashing it seems),
e.g. for `"` -> `VK_QUOTE`, which has a value of `0xDE`.

I am guessing that the reason we have not seen any bug reports on this
is that the keystroke handler falls back to an alternate code path, so
in the vast majority of cases, keyboards would continue to work
correctly. Furthermore, the test was just for truthiness of the `BOOL`
so probably at least 50% of the time we'd have been okay anyway. Or
something. Anyway, I reckon this is better.