infcmd, btrace: fix crash in 'finish' for tailcall-only frames

infcmd, btrace: fix crash in 'finish' for tailcall-only frames

Patch 7eb895307f53 Skip unwritable frames in command "finish"
skips non-writable frames in addition to tailcall frames.

If skip_tailcall_frames already returns NULL, skip_unwritable_frames
will be called with a NULL frame and crash in get_frame_arch.  This is
caught by gdb.btrace/tailcall-only.exp.

Further, if we ever end up with a mixture of tailcall and non-writable
frames, we may not skip all of them, as intended.

Loop over skip_tailcall_frames and skip_unwritable_frames as long as at least
one of them makes progress.

gdb/
	* infcmd.c (skip_finish_frames): New.
	(finish_command): Call skip_finish_frames.