Keep openssl errors associated with the right bufferevent object.

Keep openssl errors associated with the right bufferevent object.

OpenSSL has a per-thread error stack, and really doesn't like you
leaving errors on the stack.  Rather than discard the errors or force
the user to handle them, this patch pulls them off the openssl stack
and puts them on a stack associated with the bufferevent_openssl.  If
the user leaves them on the stack then, it won't affect any other
connections.

This bug was found by Roman Puls.  Thanks!

svn:r1481