Detect infinite loop in value_fetch_lazy's lval_register handling.

Detect infinite loop in value_fetch_lazy's lval_register handling.

If value_fetch_lazy loops infinitely while unwrapping lval_register
values, it means we either somehow ended up with two frames with the
same ID in the frame chain, or some code is trying to unwind behind
get_prev_frame's back (e.g., a frame unwind sniffer trying to unwind).
In any case, it should always be an internal error to end up in this
situation.

This patch adds a check and throws an internal error if the same frame
is returned.

2013-11-22  Tom Tromey  <tromey@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR backtrace/16155
	* value.c (value_fetch_lazy): Internal error if
	get_frame_register_value returns the same register.