Fix a crash if -D' is used without -t title'

dialog(3)'s dlg_reallocate_gauge(), used both by dialog(3)'s dialog_gauge()
and dialog(1)'s `--gauge', will segmentation fault in strlen(3) if no title
is set for the widget. Reproducible with `dialog --gauge hi 6 20' (adding
`--title ""' is enough to prevent segmentation fault).

MFC after: 3 days
X-MFC-to: stable/10