Initialize d_sender_id to some sentinel "invalid" value in BaseClassUnique

[rpavlik]

Would make it easier and more reliable to detect ordering issues (that is, calling things that use it before calling vrpn_BaseClass::init) - right now it's a read of an uninitialized location which is undefined behavior - so it might give you garbage, or your whole code might get optimized away.

[russell-taylor]

Added in c5429b9