Fix DTLS1_BAD_VER regression

Commit 9cf0f187

 in HEAD, and 68039af3 in 1.0.2, removed a version check
from dtls1_buffer_message() which was needed to distinguish between DTLS
1.x and Cisco's pre-standard version of DTLS (DTLS1_BAD_VER).

Based on an original patch by David Woodhouse <dwmw2@infradead.org>
RT#3703

Reviewed-by: Tim Hudson <tjh@openssl.org>