Fix initializations before ShouldNotReachHere() etc. and enable -Wuninitialized on linux.

In several places variables are initializaed in switch statements or the like, where the default case is decorated with ShouldNotReachHere() and the initialization is missing.

In the debug build, ShouldNotReachHere() can be suppressed, so the uninitialized value actually can cause problems.

In opt builds, not all tools recognize the ShouldNotReachHere properly.

This adds a row of initializations where I encountered problems and enables -Wuninitialized with linux gcc 4.8.
This would find most of these warnings. The flag works best in the product build, as it depends on compiler optimizations.

This unveiled some missing initializations in constructors, as in compile.hpp and oopMap.hpp, that can actually cause wrong behavior.