[SPARC64]: Fix put_user() calls in binfmt_aout32.c

argv and envp are pointers to u32's in userspace, so don't
try to put_user() a NULL to them.

Aparently gcc-4.2.x now warns about this, and since we use
-Werror for arch/sparc64 code, this breaks the build.

Signed-off-by: David S. Miller <davem@davemloft.net>