bfd: alpha: Fix crash caused by double free with --no-keep-memory

bfd: alpha: Fix crash caused by double free with --no-keep-memory

Without this, ld has been seen to crash in libc when freeing tsec_free:

*** Error in `/usr/bin/ld': double free or corruption (!prev): 0x0000000120ceb6a0 ***

_bfd_elf_link_read_relocs will always return the cached value if
present, even if keep_memory is false, therefore setting tsec_free to
NULL only when keep_memory is true is not sufficient.

	* elf64-alpha.c (elf64_alpha_relax_opt_call): Don't set tsec_free
	if relocs are cached.