Free sframe context memory on bfd_close

Call sframe_decoder_free in _bfd_elf_free_cached_info and correct
other calls to sframe_decoder_free so that sfd_info->stf_ctx is
cleared.  If sfd_info->stf_ctx isn't cleared we get double frees.

	* elf-sframe.c (sframe_decode): Do not pass local var sfd_ctx
	address to sframe_decoder_free, pass the actual context address.
	(_bfd_elf_merge_section_sframe): Likewise.
	* elf.c: Include sframe-api.h.
	(_bfd_elf_free_cached_info): Free sframe info.