Simplify operator bool()'s used for Tag error checks

src/file_analysis/Tag.h

@@ -42,7 +42,7 @@ public:
 	 * Returns false if the tag represents an error value rather than a
 	 * legal analyzer type.
 	 */
-	explicit operator bool() const	{ return *this != Tag(); }
+	explicit operator bool() const	{ return *this != Error; }
 
 	/**
 	 * Assignment operator.