[tls] Fix wrong memset in function tls_clear_cipher

sizeof(cipherspec) is obviously wrong in this context, because it will
only zero the first 4 or 8 bytes (cipherspec is a pointer).

This problem was reported by cppcheck.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

 			       cipherspec->pubkey_ctx );
 	}
 	free ( cipherspec->dynamic );
-	memset ( cipherspec, 0, sizeof ( cipherspec ) );
+	memset ( cipherspec, 0, sizeof ( *cipherspec ) );
 	cipherspec->suite = &tls_cipher_suite_null;
 }