Get rid of most warnings when building OSC block

[totalgee]

• these were particularly verbose on Windows, mainly related to possible loss of data during size conversions (size_t to int32_t or uint32_t).
• there are still a few other warnings from ASIO, but fixing those may require replacing/updating the ASIO library.

I also added asserts (for extra safety/correctness), but if you don't like those they can be removed. We can "probably" assume no one will try to send OSC messages, arguments or blobs that are more than 2GB or 4GB in size, but just in case... (-;

(unrelated) I also fixed a bug in the Rand unit test, where I assume it was intending to only run a performance test in Release builds, but was instead doing it in Debug (which took forever). At least on Windows, DEBUG wasn't defined in the project (although _DEBUG is) -- but the proper test is normally #ifdef NDEBUG if you want optimized (Release) builds. This should work across all platforms.

[andrewfb]

Thanks for these, including the NDEBUG fix.