The finish_body() function can raise ssl.SSLWantReadError when
discarding unread request body data on SSL connections. This causes
TLS requests to fail intermittently with "Invalid request" errors.
Handle SSLWantReadError by treating it as "no more data to read".
This is safe because finish_body() only discards leftover data before
keepalive - if SSL says "need to wait for more data", there's nothing
left to discard.
Fixes#3448