connected: close err_fd in promisor fast-path

connected.h documents that err_fd is closed before check_connected()
returns. It is, on three of four exit paths. The promisor-pack fast
path added in 50033772d (connected: verify promisor-ness of partial
clone, 2020-01-30) returns 0 without closing it.

receive-pack uses err_fd as the write end of an async sideband
muxer's pipe, and the muxer thread waits for EOF. The same omission
has caused deadlocks there twice before: 49ecfa13f (receive-pack:
close sideband fd on early pack errors, 2013-04-19) and 6cdad1f13
(receive-pack: fix deadlock when we cannot create tmpdir,
2017-03-07).

Signed-off-by: Ethan Dickson <ethanndickson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

connected.c

@@ -91,6 +91,8 @@ promisor_pack_found:
 			;
 		} while ((oid = fn(cb_data)) != NULL);
 		free(new_pack);
+		if (opt->err_fd)
+			close(opt->err_fd);
 		return 0;
 	}