Sanity check the return from final_finish_mac

The return value is checked for 0. This is currently safe but we should
really check for <= 0 since -1 is frequently used for error conditions.
Thanks to Kevin Wojtysiak (Int3 Solutions) and Paramjot Oberoi (Int3
Solutions) for reporting this issue.
Reviewed-by: NAndy Polyakov <appro@openssl.org>

ssl/s3_both.c

@@ -168,7 +168,7 @@ int ssl3_send_finished(SSL *s, int a, int b, const char *sender, int slen)
         i = s->method->ssl3_enc->final_finish_mac(s,
                                                   sender, slen,
                                                   s->s3->tmp.finish_md);
-        if (i == 0)
+        if (i <= 0)
             return 0;
         s->s3->tmp.finish_md_len = i;
         memcpy(p, s->s3->tmp.finish_md, i);

ssl/ssl_locl.h

@@ -2073,7 +2073,6 @@ void dtls1_set_message_header(SSL *s,
 __owur int dtls1_write_app_data_bytes(SSL *s, int type, const void *buf, int len);
 
 __owur int dtls1_send_change_cipher_spec(SSL *s, int a, int b);
-__owur int dtls1_send_finished(SSL *s, int a, int b, const char *sender, int slen);
 __owur int dtls1_read_failed(SSL *s, int code);
 __owur int dtls1_buffer_message(SSL *s, int ccs);
 __owur int dtls1_retransmit_message(SSL *s, unsigned short seq,