warn when UNICODE_STRING fields are not all set properly

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on July 28, 2011 12:03:28_

xref issue #490 where kernel doesn't set Length for some OUT UNICODE_STRINGs but
does for most.
xref issue #516 where user libs don't set MaximumLength for some IN
UNICODE_STRINGs sent to kernel.
yet both fields are set often enough that there's something sloppy here:
either the documentation is sloppy about what's required, or the
conventions aren't being followed.
we should figure it out and perhaps issue warnings when both fields aren't
set (if the convention really is to set both), but not errors.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=519_