In socks5_connect(), do not try to print a negative number ...

In socks5_connect(), do not try to print a negative number ...

... of socks response bytes if the read() for the socks5
negotiation failed completely.

It's an API violation and can trigger an assert() in
debug builds with "debug 2" enabled. Except for the
assert(), log_error() already handled negative %N values
gracefully so this commit only makes a difference for
debug builds.