Gmt module: Fix error in conversion function from string to gmt_dash.

modules/gmt/modgmt_structs.h

@@ -720,8 +720,8 @@ struct gmt_dash: public gmt_struct
   // Determine shift
   {
    WordList wl=Split(in,":");
-   if(wl.size()>1) return false;
+   if(wl.size()>2) {err="Incorrect dash format"; return false;}
-   if(1==wl.size())
+   if(2==wl.size())
    {
     Value2Width s(wl.back());
     bool suc=true;