Fix using speak/speak_active without a text beforehand

Otherwise, it would end up indexing a vector out-of-bounds, which is UB
and causes a segfault, too. This is used in some constructs like

    say(-1)
    <internal command>

where the text contents don't matter, only that a text box shows up.

desktop_version/src/Script.cpp

@@ -577,6 +577,10 @@ void scriptclass::run()
 			{
 				//Ok, actually display the textbox we've initilised now!
 				//If using "speak", don't make the textbox active (so we can use multiple textboxes)
+				if (txt.empty())
+				{
+					txt.resize(1);
+				}
 				graphics.createtextbox(txt[0], textx, texty, r, g, b);
 				if ((int) txt.size() > 1)
 				{