Fix ptype/o bug with "<no data fields>"

Fix ptype/o bug with "<no data fields>"

I noticed that when using ptype/o, the "<no data fields>" text that
may be emitted is indented incorrectly.  This patch fixes the bug and
adds a new test case.

I also removed a stray backslash from ptype-offsets.exp that I noticed
while writing the test.  This seemed too trivial to warrant a separate
patch.

gdb/ChangeLog
2021-04-22  Tom Tromey  <tom@tromey.com>

	* c-typeprint.c (c_type_print_base_struct_union): Use
	print_spaces_filtered_with_print_options.

gdb/testsuite/ChangeLog
2021-04-22  Tom Tromey  <tom@tromey.com>

	* gdb.base/ptype-offsets.cc (struct empty_member): New.
	(main): Use empty_member.
	* gdb.base/ptype-offsets.exp: Add new test.