bsddialog(3): fix disabled shadow

[asiciliano]

Summary

Fix dialogs building and updating segmentation fault with disabled shadow (conf.shadow=false and implicitly bsddialog --no-shadow).
No component in BASE is affected.
Moreover delete WARNS: all warnings were fixed in upstream and imported in 0.2.

Test Plan

Trivial:

-       if (shadow != NULL)
+       if (conf->shadow)

(Probably the error was introduced during the last refactoring to improve the dialogs building process).