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

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