Don't call sendfile() if count=0. With FreeBSD this fixes an assert crash.
Don't call sendfile() if count=0. With FreeBSD this fixes an assert crash.

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branch : HEAD

dscho/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,dscho/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,dscho/dovecot,jwm/dovecot-notmuch,dscho/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,dscho/dovecot