ast: add a null check

libsolidity/ast/AST.cpp

@@ -189,6 +189,7 @@ vector<Declaration const*> const& ContractDefinition::inheritableMembers() const
 		m_inheritableMembers.reset(new vector<Declaration const*>());
 		auto addInheritableMember = [&](Declaration const* _decl)
 		{
+			solAssert(_decl, "addInheritableMember got a nullpointer.");
 			if (memberSeen.count(_decl->name()) == 0 && _decl->isVisibleInDerivedContracts())
 			{
 				memberSeen.insert(_decl->name());