tls: fix NULL deref on tls_sw_splice_eof() with empty record

In the Linux kernel, the following vulnerability has been resolved:

tls: fix NULL deref on tls_sw_splice_eof() with empty record

syzkaller discovered that if tls_sw_splice_eof() is executed as part of

sendfile() when the plaintext/ciphertext sk_msg are empty, the send path

gets confused because the empty ciphertext buffer does not have enough

space for the encryption overhead. This causes tls_push_record() to go on

the `split = true` path (which is only supposed to be used when interacting

with an attached BPF program), and then get further confused and hit the

tls_merge_open_record() path, which then assumes that there must be at

least one populated buffer element, leading to a NULL deref.

It is possible to have empty plaintext/ciphertext buffers if we previously

bailed from tls_sw_sendmsg_locked() via the tls_trim_both_msgs() path.

tls_sw_push_pending_record() already handles this case correctly; let's do

the same check in tls_sw_splice_eof().