Add a useful assert in the comparison engine

* src/abg-comparison.cc (qualified_type_diff::report): Assert that
	if the qualified type diff node has changes to be reported and no
	local change, then its child node must have changes to be
	reported.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>

src/abg-comparison.cc

@@ -5643,6 +5643,7 @@ qualified_type_diff::report(ostream& out, const string& indent) const
 
   diff_sptr d = leaf_underlying_type_diff();
   assert(d);
+  assert(d->to_be_reported());
   RETURN_IF_BEING_REPORTED_OR_WAS_REPORTED_EARLIER2(d,
 						    "unqualified "
 						    "underlying type");