Bug#42675: Dangling pointer leads to a client crash (mysys/my_error.c \

	patch enclosed)
  
One call to my_error_unregister_all() would free pointers, but leave one
pointer to just-freed memory still assigned.  That's the bug.  Subsequent
calls of this function would try to follow pointers into deallocated, 
garbage memory and almost certainly SEGV.

Now, after freeing a linked list, unset the initial pointer.

mysys/my_error.c

@@ -252,11 +252,16 @@ const char **my_error_unregister(int first, int last)
 
 void my_error_unregister_all(void)
 {
-  struct my_err_head    *list, *next;
-  for (list= my_errmsgs_globerrs.meh_next; list; list= next)
+  struct my_err_head *cursor, *saved_next;
+
+  for (cursor= my_errmsgs_globerrs.meh_next; cursor != NULL; cursor= saved_next)
   {
-    next= list->meh_next;
-    my_free((uchar*) list, MYF(0));
+	/* We need this ptr, but we're about to free its container, so save it. */
+    saved_next= cursor->meh_next;
+
+    my_free((uchar*) cursor, MYF(0));
   }
+  my_errmsgs_globerrs.meh_next= NULL;  /* Freed in first iteration above. */
+
   my_errmsgs_list= &my_errmsgs_globerrs;
 }