When we're reading out plaintext from rustls' internal buffers, we might
get a read of zero bytes (meaning a clean TCP close, including
close_notify). However, we shouldn't return immediately when that
happens, since we may have already copied out some plaintext bytes.
Break out of the loop when we get a read of zero bytes, and figure out
which path we're dealing with.
Acked-by: Kevin Burke
Closes#8003
