Crash in a nice way if the GenericEnv is NULL

On every `pushScope` the generic environment of this scope is appended
to `GenericEnv`.  If the pushed `DeclContext` isn't generic, `NULL` is
pushed leading to a crash only in `DenseMap::find` if some archetypes
escaped their generic context.

Author: JaSpa

lib/AST/ASTVerifier.cpp

@@ -437,7 +437,7 @@ struct ASTNodeBase {};
           }
 
           // Otherwise, the archetype needs to be from this scope.
-          if (GenericEnv.empty()) {
+          if (GenericEnv.empty() || !GenericEnv.back()) {
             Out << "AST verification error: archetype outside of generic "
                    "context: " << archetype->getString() << "\n";
             return true;