Avoid segfault when an IMAP connection cannot be opened (II)

Avoid segfault when an IMAP connection cannot be opened (II)

Part II of 6a2e9f, which broke edit-msg. It turns out that mx_ac_add()
is not required to be successful, e.g., when opening a temporary file.
This means we must be extra cautious and check that m->account->adata is
valid before using it