s3:auth:sam_password_ok: fix allocation of a data blob.

s3:auth:sam_password_ok: fix allocation of a data blob.

data_blob(mem_ctx, 16) does not use mem_ctx as a talloc ctx but
copies 16 bytes from mem_ctx into the newly allocated data blob.
This can not have been intentional. A blank uint8_t array of
length 16 is allocated by passing NULL instead of mem_ctx.
And using data_blob_talloc(mem_ctx, NULL, 16) adds the allocated
blank 16 byte array to mem_ctx - so this is what must have been
intended.

Michael