drsyscall reports zero-length memory blobs on Windows

_From [rnk@google.com](https://code.google.com/u/rnk@google.com/) on March 14, 2013 16:28:16_

I added an assert after observing one of these memargs from drsyscall on Linux, but it fires all the time during the suite on Windows, so I'm disabling it for Windows. This issue covers fixing up the Windows drsyscall handlers and tables to avoid reporting zero length memory blobs.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1171_