Add bounds check to setlinegap

This is for consistency with all other functions dealing with the latest
created text box. There are several cases in custom levels where these
functions can be called even though there are no text boxes on screen.

desktop_version/src/Graphics.cpp

@@ -1508,6 +1508,12 @@ void Graphics::textboxadjust(void)
 
 void Graphics::setlinegap(int customvalue)
 {
+    if (!INBOUNDS_VEC(m, textboxes))
+    {
+        vlog_error("setlinegap() out-of-bounds!");
+        return;
+    }
+
     textboxes[m].linegap = customvalue;
     textboxes[m].resize();
 }