Don't drain extra data when parsing socks auth methods

We added this back in 0649fa14 in 2006, to deal with the case where
the client unconditionally sent us authentication data.  Hopefully,
that's not needed any longer, since we now can actually parse
authentication data.

src/or/buffers.c

@@ -1736,9 +1736,9 @@ parse_socks(const char *data, size_t datalen, socks_request_t *req,
           req->reply[1] = '\xFF'; /* reject all methods */
           r=-1;
         }
-        /* remove packet from buf. also remove any other extraneous
-         * bytes, to support broken socks clients. */
-        *drain_out = -1;
+        /* Remove packet from buf. Some SOCKS clients will have sent extra
+         * junk at this point; let's hope it's an authentication message. */
+        *drain_out = 2u + nummethods;
 
         return r;
       }