imap-send: avoid buffer overflow

imap-send: avoid buffer overflow

We format the password prompt in an 80-character static
buffer. It contains the remote host and username, so it's
unlikely to overflow (or be exploitable by a remote
attacker), but there's no reason not to be careful and use
a strbuf.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>