gdb/cli: Remove casts of NULL during assignment.

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.