gdb: Remove casts of NULL during assignment.

gdb: 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/ directory where we
cast NULL during assignment, removing the cast.

gdb/ChangeLog:

	* c-valprint.c (print_unpacked_pointer): Remove cast of NULL
	pointer.
	* dbxread.c (dbx_end_psymtab): Likewise.
	* gnu-nat.c (gnu_write_inferior): Likewise.
	* mdebugread.c (cross_ref): Likewise.
	* p-valprint.c (pascal_val_print): Likewise.
	* xcoffread.c (xcoff_end_psymtab): Likewise.