bug#10843: 23.4; Fix gnus-group-post-news error

[Leo]

On a topic line in the group buffer, key C-u a, to get an error. After
this, every time you enter a group you get an error like this:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match(".*" nil)
  gnus-group-name-charset(nil nil)
  gnus-group-decoded-name(nil)
  gnus-summary-buffer-name(nil)
  gnus-article-setup-buffer()
  gnus-select-newsgroup("gmane.comp.lang.scala" 1 nil)
  gnus-summary-read-group-1("gmane.comp.lang.scala" 1 t nil nil nil)
  gnus-summary-read-group("gmane.comp.lang.scala" 1 t nil nil nil nil)
  gnus-group-read-group(1 t)
  gnus-group-select-group(1)
  gnus-topic-select-group(1)
  call-interactively(gnus-topic-select-group nil nil)

The following patch fixes this bug.

=== modified file 'lisp/gnus/gnus-msg.el'
--- lisp/gnus/gnus-msg.el       2012-02-09 23:07:49 +0000
+++ lisp/gnus/gnus-msg.el       2012-02-19 08:07:22 +0000
@@ -636,12 +636,12 @@
   (interactive "P")
   ;; Bind this variable here to make message mode hooks work ok.
   (let ((gnus-newsgroup-name
-        (if arg
-            (if (= 1 (prefix-numeric-value arg))
-                (gnus-group-completing-read "Newsgroup" nil
-                                            (gnus-read-active-file-p))
-              (gnus-group-group-name))
-          ""))
+        (or (and arg
+                 (if (= 1 (prefix-numeric-value arg))
+                     (gnus-group-completing-read "Newsgroup" nil
+                                                 (gnus-read-active-file-p))
+                   (gnus-group-group-name)))
+            ""))
        ;; make sure last viewed article doesn't affect posting styles:
        (gnus-article-copy))
     (gnus-post-news 'post gnus-newsgroup-name nil nil nil nil