Fix Semantic tag discovery when loading of unloaded files is suppressed

Fix Semantic tag discovery when loading of unloaded files is suppressed

* lisp/cedet/semantic/db-find.el
(semanticdb-find-tags-by-name-method)
(semanticdb-find-tags-by-name-regexp-method)
(semanticdb-find-tags-for-completion-method)
(semanticdb-find-tags-by-class-method)
(semanticdb-find-tags-external-children-of-type-method)
(semanticdb-find-tags-subclasses-of-type-method)
(semanticdb-deep-find-tags-by-name-method)
(semanticdb-deep-find-tags-by-name-regexp-method)
(semanticdb-deep-find-tags-for-completion-method): Verify that
tags is bound before accessing it; this slot is unbound in
tables created for unloaded files when the `unloaded' throttle
is disabled.

(cherry picked from commit 3968b36ae0641e929426991028b49ce66a15af5f)