Fix a double-free in failing case of handle_control_authenticate.

Bugfix on ed8f020e205267e6270494634346ab68d830e1d8; bug not in any
released version of Tor.  Found by Coverity; this is CID 1239290.

[Yes, I used this commit message before, in 58e813d0fc.
Turns out, that fix wasn't right, since I didn't look up a
screen. :P ]

src/or/control.c

@@ -1173,6 +1173,7 @@ handle_control_authenticate(control_connection_t *conn, uint32_t len,
       bad_password = 1;
       SMARTLIST_FOREACH(sl, char *, cp, tor_free(cp));
       smartlist_free(sl);
+      sl = NULL;
     } else {
       SMARTLIST_FOREACH(sl, char *, expected,
       {