Free ecoff debug info

This frees memory associated with the mips ecoff find_nearest_line.

	* elfxx-mips.x (free_ecoff_debug): New function, extracted from..
	(_bfd_mips_elf_read_ecoff_info): ..here.  Free ext_hdr earlier.
	Don't clear already NULL fdr.
	(struct mips_elf_find_line): Move earlier.
	(_bfd_mips_elf_close_and_cleanup): Call free_ecoff_debug.
	(_bfd_mips_elf_find_nearest_line): Likewise on error paths,
	and to clean up input_debug when done.