imap-send: properly error out if imap.host is not set in config

imap-send: properly error out if imap.host is not set in config

If no imap host is specified in the git config, git imap-send used
to try to lookup a null pointer through gethostbyname(), causing a
segfault.  Since setting the imap.host variable is mandatory,
imap-send now properly fails with an explanatory error message.

The problem has been reported by picca through
 http://bugs.debian.org/472632

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>