btrace: no replay without history

When using a reverse execution command without execution history, GDB
might end up in a state where replaying has been started but remains
at the current instruction.  This state is illegal.

Do not step if there is no execution history to avoid this.

2014-05-20  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_step_thread): Check for empty history.

testsuite/
	* gdb.btrace/nohist.exp: New.