Remove warning about a loop parsing evbuffer socks

This behavior is normal when we want more data than the evbuffer
actually has for us.  We'll ask for (say) 7 bytes, get only 5
(because that's all there is), try to parse the 5 bytes, and get
told "no, I want 7".  One option would be to bail out early whenever
want_length is > buflen, but sometimes we use an over-large
want_length.  So instead, let's just remove the warning here: it's
not a bug after all.