Fix crash when using recent versions of wxsqlite3

Recent versions of wxsqlite3 throw an exception if a database is not
open when calling the Close method. This change adds a check to
Codelite/tags_storage_sqlite3.h to not call the method if no database
is open, thus preventing the exception.
Merge pull request #540 from jcowgill/fix-wxsqlite3-crash

Fix crash when using recent versions of wxsqlite3 (James Cowgill)