Don't add copied scale vector as new vector, but as new scale vector,

thus preventing crash after pushing X11 'Hardcopy' twice or
pushing 'quit' after 'Hardcopy'.
Scale vector was deleted two times, then crashimg ngsoice.

src/frontend/plotting/graphdb.c

@@ -157,11 +157,7 @@ GRAPH *CopyGraph(GRAPH *graph)
                     new_plotdata = newlink; /* put in front */
                     struct dvec * const new_scale = vec_copy(old_scale);
                     new_scale->v_flags |= VF_PERMANENT;
-                    newlink = TMALLOC(struct dveclist, 1);
-                    newlink->next = new_plotdata;
-                    newlink->f_own_vector = TRUE;
-                    newlink->vector = new_scale;
-                    newlink->next = new_plotdata;
+                    newlink->vector->v_scale = new_scale;
                 }
             }
             else {
@@ -259,12 +255,13 @@ int DestroyGraph(int id)
                     nextd = d->next;
                     if (d->f_own_vector) {
                         /* list responsible for freeing this vector */
-                        if (d->vector->v_scale)
-                            dvec_free(d->vector->v_scale);
+                        if (d->vector->v_scale) {
+                            dvec_free(d->vector->v_scale);  
+                        }                          
                         dvec_free(d->vector);
                     }
-                    txfree(d);
-                    d = nextd;
+                    txfree(d);                    
+                    d = nextd;                    
                 }
             }