pass NULL to TARGET_DEFAULT_RETURN when appropriate

pass NULL to TARGET_DEFAULT_RETURN when appropriate

This changes instances of TARGET_DEFAULT_RETURN(0) to
TARGET_DEFAULT_RETURN(NULL) when appropriate.  The use of "0" was a
relic from an earlier implementation of make-target-delegates; and I
didn't want to go back through the long patch series, fixing up
conflicts, just to change this small detail.

2014-02-19  Tom Tromey  <tromey@redhat.com>

	* target-delegates.c: Rebuild.
	* target.h (struct target_ops) <to_extra_thread_info,
	to_thread_name, to_pid_to_exec_file, to_get_section_table,
	to_memory_map, to_read_description, to_traceframe_info>: Use NULL,
	not 0, in TARGET_DEFAULT_RETURN.