Add assertions to the [c|r]mutex related functions on Windows

This is required to achieve the behavior specified by the Unix functions and is expected from the source code that calls these functions.

It should be noted that if DBus is built without assertions, these functions will no longer match the behavior expected by the calling functions in case of returned errors.

This problem is not limited to Windows builds.

Fixes #369 (closed)

Depends on !192 (merged)