[tls] Tidy up error handling flow in tls_send_plaintext()

[tls] Tidy up error handling flow in tls_send_plaintext()

Coverity reported that tls_send_plaintext() failed to check the return
status from tls_generate_random(), which could potentially result in
uninitialised random data being used as the block initialisation
vector (instead of intentionally random data).

Add the missing return status check, and separate out the error
handling code paths (since on the successful exit code path there will
be no need to free either the plaintext or the ciphertext anyway).

Signed-off-by: Michael Brown <mcb30@ipxe.org>