gdb/

	gdb/
	* tracepoint.c (tfile_xfer_partial): If there's no traceframe
	selected, don't try iterating over the traceframe's blocks.
	(tfile_has_stack): If there's no traceframe selected, then there's
	no stack.
	(tfile_has_registers): If there's no traceframe selected, then
	there's no registers.

	gdb/testsuite/
	* gdb.trace/tfile.exp: Test that with no traceframe selected,
	there's no stack or registers.