imap: fix oauth

imap: fix oauth

Currently when oauth success, we got this and neomutt UI got stuck:

[2019-04-21 15:35:38]<5> mutt_socket_readln_d() 5< a0001 OK xxxxxxxxxxxxx@xxxxxxxxx authenticated (Success)
[2019-04-21 15:35:38]<2> mutt_socket_write_d() 5> [2019-04-21 15:35:38]<2> imap_login() Communication encrypted at 256 bits
[2019-04-21 15:35:38]<3> imap_mdata_new() hcache uidvalidity 1, uidnext 228, modseq 0
[2019-04-21 15:35:38]<M> mx_mbox_open() Reading imaps://imap.gmail.com/INBOX...
[2019-04-21 15:35:38]<M> imap_mbox_open() Selecting INBOX...
[2019-04-21 15:35:38]<2> mutt_socket_write_d() 5> a0002 SELECT "INBOX"
[2019-04-21 15:35:38]<5> mutt_socket_readln_d() 5< * BAD [CLIENTBUG] Invalid tag

In fact neomutt send ^A to the server, but this should occurs only if
the authentication fails.

This changes fixes that.