Special case NULL when using printf's %s format

Special case NULL when using printf's %s format

This changes the printf command's %s and %ls formats to special-case
NULL, and print "(null)" for these.  This is PR cli/14977.  This
behavior seems a bit friendlier; I was undecided on whether other
invalid pointers should be handled specially somehow, so for the time
being I've left those out.

gdb/ChangeLog
2018-03-14  Tom Tromey  <tom@tromey.com>

	PR cli/14977:
	* printcmd.c (printf_c_string, printf_wide_c_string): Special case
	for NULL.

gdb/gdbserver/ChangeLog
2018-03-14  Tom Tromey  <tom@tromey.com>

	PR cli/14977:
	* ax.c (ax_printf): Special case for NULL.

gdb/testsuite/ChangeLog
2018-03-14  Tom Tromey  <tom@tromey.com>

	PR cli/14977:
	* gdb.base/printcmds.exp (test_printf): Add printf test of %s with
	a null pointer.
	* gdb.base/wchar.exp: Likewise.