Applying same fix as in dtls1_process_out_of_seq_message. A truncated DTLS...

Applying same fix as in dtls1_process_out_of_seq_message. A truncated DTLS fragment would cause *ok to be clear, but the return value would still be the number of bytes read.

Problem identified by Emilia Käsper, based on previous issue/patch by Adam
Langley.

Reviewed-by: Emilia Käsper <emilia@openssl.org>