btrace, frame: fix crash in get_frame_type

btrace, frame: fix crash in get_frame_type

In skip_artificial_frames we repeatedly call get_prev_frame_always until we get
a non-inline and non-tailcall frame assuming that there must be such a frame
eventually.

For record targets, however, we may have a frame chain that consists only of
artificial frames.  This leads to a crash in get_frame_type when dereferencing a
NULL frame pointer.

Change skip_artificial_frames and skip_tailcall_frames to return NULL in such a
case and modify each caller to cope with a NULL return.

In frame_unwind_caller_pc and frame_unwind_caller_arch, we simply assert that
the returned value is not NULL.  Their caller was supposed to check
frame_unwind_caller_id before calling those functions.

In other cases, we thrown an error.

In infcmd further move the skip_tailcall_frames call to the forward-stepping
case since we don't need a frame for reverse execution and we don't want to fail
because of that.  Reverse-finish does make sense for a tailcall frame.

gdb/
	* frame.h (skip_tailcall_frames): Update comment.
	* frame.c (skip_artificial_frames, skip_tailcall_frames): Return NULL
	if only	artificial frames are found.  Update comment.
	(frame_unwind_caller_id): Handle NULL return.
	(frame_unwind_caller_pc, frame_unwind_caller_arch): Assert that
	skip_artificial_frames does not return NULL.
	(frame_pop): Add an error if only tailcall frames are found.
	* infcmd.c (finish_command): Move skip_tailcall_frames call into forward-
	execution case.  Add an error if only tailcall frames are found.

testsuite/
	* gdb.btrace/tailcall-only.exp: New.
	* gdb.btrace/tailcall-only.c: New.
	* gdb.btrace/x86_64-tailcall-only.S: New.
	* gdb.btrace/i686-tailcall-only.S: New.