fast-import.c: don't try to commit marks file if write failed

We also move the assignment of -1 to the lock file descriptor
up, so that rollback_lock_file() can be called safely after a
possible attempt to fclose(). This matches the contents of
the 'if' statement just above testing success of fdopen().

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>