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 58e813d0fcfcecfc2017.
Turns out, that fix wasn't right, since I didn't look up a
screen. :P ]