gdb/cli: Remove casts of NULL during assignment.

In the following code:

    struct symbol *wsym = (struct symbol *) NULL;

the cast of NULL is redundant, it adds noise, and is just one more thing
to change if the type of wsym ever changes.  There are a relatively
small number of places in gdb where the above code pattern is used.
Usually the cast is removed like this:

    struct symbol *wsym = NULL;

This commit updates all the places within the gdb/cli directory where we
cast NULL during assignment, removing the cast.

gdb/ChangeLog:

	* cli/cli-decode.c (find_cmd): Remove cast of NULL pointer.

gdb/cli/cli-decode.c

@@ -1219,7 +1219,7 @@ find_cmd (const char *command, int len, struct cmd_list_element *clist,
 {
   struct cmd_list_element *found, *c;
 
-  found = (struct cmd_list_element *) NULL;
+  found = NULL;
   *nfound = 0;
   for (c = clist; c; c = c->next)
     if (!strncmp (command, c->name, len)