GuiBox.cpp: addendum to [f04c7711/lyxgit]

- this was forgotten to commit;
(without this we would get invalid LaTeX code when changing in the box dialog a makebox without a width to a minipage and press apply - a makebox required to specify a width, at least an invisible one like "0cm")

src/frontends/qt4/GuiBox.cpp

@@ -378,6 +378,9 @@ docstring GuiBox::dialogToParams() const
 			params.width = Length(value.toDouble(), Length::IN);
 		} else {
 			params.special = "none";
+			// we must specify a valid length in this case
+			if (value.isEmpty())
+				widthED->setText("0");
 			params.width = Length(widgetsToLength(widthED, widthUnitsLC));
 		}
 	} else {