TLSProxy/Proxy.pm: refine partial packet handling.

Original logic was "if no records found *or* last one is truncated, then
leave complete records in queue." Trouble is that if we don't pass on
complete records and get complete packet in opposite direction, then
queued records will go back to sender. In other words complete records
should always be passed on. [Possible alternative would be to match
direction in reconstruct_record.]

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5887)