Gmt module: Fix error in gmt_font::Convert().

modules/gmt/modgmt_structs.h

@@ -828,11 +828,12 @@ struct gmt_font: public gmt_struct
  {
   WordList::const_iterator ci;
   WordList wl=Split(str,",",true);
+  std::string fakeerr;
 
   // Defaults
   size=default_size;
   family=default_family;
-  color.Convert(0,err); // Black
+  color.Convert(0,fakeerr); // Black
 
   if(wl.size()>3) {err="String "+str+" is not font string"; return false;} // String is [size][,family][,color] or [family][,color]
   ci=wl.begin();
@@ -840,7 +841,11 @@ struct gmt_font: public gmt_struct
   {
    Value2Width s(*ci);
    bool suc=true;
-   {std::string fake; size=s(&suc,fake);}
+   {
+    std::string fake;
+    double newsize=s(&suc,fake);
+    if(suc) size=newsize;
+   }
    if(!suc) // Parse error. check if argument is font name
    {
     if(0==families.count(*ci)) return false; // No, argument is not allowed font name