work around a segmentation fault, plot .. vs vec

ChangeLog

@@ -1,3 +1,9 @@
+2010-07-31  Robert Larice
+ * src/frontend/parse.c :
+    work around a segmentation fault, when plotting
+      plot something vs vec
+    vec was freed too early
+
 2010-07-31  Robert Larice
  * src/frontend/vectors.c :
     fix an ancient typo, responsible for segfault or garbage

src/frontend/parse.c

@@ -416,7 +416,7 @@ free_pnode_x(struct pnode *t)
 	free_pnode(t->pn_right);
 	free_pnode(t->pn_next);
 	tfree(t->pn_name); /* va: it is a copy() of original string, can be free'd */
-	if (t->pn_value)
+	if (t->pn_value && !(t->pn_value->v_flags & VF_PERMANENT))
 	    vec_free(t->pn_value); /* patch by Stefan Jones */
 	tfree(t);
     }