WL#1905: Make sure NDB code compiles without warnings on Linux

Affects: Server-4.1   —   Status: Un-Assigned   —   Priority: Low

As per our general coding guidelines, code should normally compile without any
warnings. This so that it becomes easier to spot any real problems.
This should at least apply for a basic Linux build.

In the case of NDB, fixing the code to not generate compiler warnings will
probably also pre-empt issues with compiling on other platforms (including 64-bit).