libelf: Don't crash elf[32|64]_xlateto[fm] on bad arguments.

Check that src and dest aren't NULL and that src->d_type < ELF_T_NUM.

      * elf32_xlatetof.c (elfw2(LIBELFBITS, xlatetof)): Check src, dest
      and src->d_type are valid.
      * elf32_xlatetom.c (elfw2(LIBELFBITS, xlatetom)): Likewise.

Signed-off-by: Mark Wielaard <mark@klomp.org>