Fix throwing nil exceptions.

[ngrewe]

The following is perfectly legal on Mac OS X:

@try
{
  @throw(nil);
}
@catch (id obj)
{
  printf("Caught: %p", obj);
}

It doesn't work presently because we were trying to dereference the object pointer to determine the class.

[davidchisnall]

What's the correct behaviour for, throwing nil and catching (for example)NSObject *? Should nil also be caught by void* catches in Objective-C++ (this case used to crash the Apple runtime, not sure if they've fixed it now)?

[ngrewe]

What's the correct behaviour for, throwing nil and catching (for example)NSObject *?

It is only caught by @catch(id) and @catch(...), not by any other interface type. I've added a few more tests for those cases, but I'm running into some weird behaviour here: The standalone test case passes, but similar code, linked against gnustep-base doesn't work (i.e. @catch(NSObject*) catches nil as well…)

Should nil also be caught by void* catches in Objective-C++ (this case used to crash the Apple runtime, not sure if they've fixed it now)?

I've checked and it's not caught by a void* handler on 10.10.5 (it doesn't crash either, fwiw).

[davidchisnall]

The stand-alone code may be interacting oddly with the CXXException.mm stuff in -base, which attempts to box C++ exceptions as Objective-C objects. We should probably remove that now - it was largely a work-around for the fact that we didn't have working Objective-C++.