Prevent assertion failure when trying to complete the prompt

lisp/ChangeLog

+2014-04-01  Daniel Colascione  <dancol@dancol.org>
+
+	* minibuffer.el (minibuffer-complete): Prevent assertion failure
+	when trying to complete the prompt.
+
 2014-03-31  Leo Liu  <sdl.web@gmail.com>
 	* emacs-lisp/eldoc.el (eldoc-print-current-symbol-info): Refactor

lisp/minibuffer.el

@@ -1092,9 +1092,10 @@ If no characters can be completed, display a list of possible completions.
 If you repeat this command after it displayed such a list,
 scroll the window of possible completions."
   (interactive)
-  (completion-in-region (minibuffer-prompt-end) (point-max)
-                        minibuffer-completion-table
-                        minibuffer-completion-predicate))
+  (when (<= (minibuffer-prompt-end) (point))
+    (completion-in-region (minibuffer-prompt-end) (point-max)
+                          minibuffer-completion-table
+                          minibuffer-completion-predicate)))
 
 (defun completion--in-region-1 (beg end)
   ;; If the previous command was not this,