Fix obvious bug in aggregate expression

Fix obvious bug in aggregate expression

I found an obvious bug in Ada aggregate expression handling:

	  if (vvo != nullptr)
 	    error (_("Invalid record component association."));
 	  name = vvo->get_symbol ()->natural_name ();

Here the code errors when vvo is not null -- and then proceeds to use
vvo.

This hasn't caused a crash because, I believe, there's currently no
way to reach this code in the null case.  However, I'm not really
willing to assert this...

Fixing this shows another bug, which is that due to the way the parser
works, a field name in an aggregate expression might erroneously be
fully qualified if some global variable with the same base name
exists.

The included test case triggers both bugs.  Note that the test
includes a confounding case for array aggregates as well, but as these
are harder to fix, I've left it as kfail.

As this is Ada-specific, and has already been tested internally at
AdaCore, I am checking it in.