bug fix, avoid $curplot (plot_names) being assembled from negative numbers

ChangeLog

@@ -1,3 +1,13 @@
+2011-06-02  Robert Larice
+ * src/frontend/postcoms.c :
+   bug fix, avoid $curplot (plot_names) being assembled from negative numbers
+     The variable `plot_num' is exclusively used to build plotnames,
+       via sprintf(plot_name, "dc%d", plot_num) expressions
+     Thus plot_num may never be decremented.
+     Otherwise expressions of sort
+	dc-2.var
+     can't be parsed
+
 2011-06-02  Robert Larice
  * src/frontend/plotting/graf.c ,
  * src/frontend/plotting/x11.c :

src/frontend/postcoms.c

@@ -800,7 +800,6 @@ com_destroy(wordlist *wl)
             {
                 DelPlotWindows(pl);
                 killplot(pl);
-                plot_num--;
             }
             else
                 fprintf(cp_err, "Error: no such plot %s\n", wl->wl_word);