avoid double free in _bfd_stab_section_find_nearest_line

This corrects the error return path after reloc_vector has been freed,
and ensures this won't happen even if the wrong goto was used.

	PR binutils/34573
	* syms.c (_bfd_stab_section_find_nearest_line): Set reloc_vector
	to NULL after freeing it.  Correct error goto.