BUGFIX: deselecting all markers freezes the gui.

A index check on current_marker value was missing.

plot.c

@@ -1493,6 +1493,8 @@ draw_all(bool flush)
 void
 redraw_marker(int marker, int update_info)
 {
+  if (marker < 0)
+    return;
   // mark map on new position of marker
   markmap_marker(marker);