x86/sgx: Return -EINVAL on a zero length buffer in sgx_ioc_enclave_add_pages()

The sgx_enclave_add_pages.length field is documented as

 * @length:     length of the data (multiple of the page size)

Fail with -EINVAL, when the caller gives a zero length buffer of data
to be added as pages to an enclave. Right now 'ret' is returned as
uninitialized in that case.

 [ bp: Flesh out commit message. ]

Fixes: c6d26d37 ("x86/sgx: Add SGX_IOC_ENCLAVE_ADD_PAGES")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/linux-sgx/X8ehQssnslm194ld@mwanda/
Link: https://lkml.kernel.org/r/20201203183527.139317-1-jarkko@kernel.org