TLSRecv clarifications

Here is a round of clean-ups triggered by @ediosyncratic's comments about whether SSL_read() can return partial reads.

My verdict is that yes this is legal (and as a result it is legal for TLSRecv() as well), and this will be the case if the opposite side did an SSL_write() of smaller size than what we are reading, or smaller than TLS record size (16KB).

But given that we are using SSL_MODE_AUTO_RETRY and that our SendTransaction() never sends more than 4KB of payload (which is smaller than the TLS record size), ReceiveTransaction() should never get partial transaction payload.
There's really no need to clear the first byte.  Either `snprintf()` succeeds (in which case it over-writes this byte) or it fails, which you check for and don't access workbuf.