dropbear: add upstream security fix for CVE-2018-15599

dropbear is affected by an user enumeration vulnerability similar to the
recent issue in openssh (CVE-2018-15473).  Add an upstream patch fixing the
issue.

For more details, see the discussion on the mailing list:
http://lists.ucc.gu.uwa.edu.au/pipermail/dropbear/2018q3/002110.html



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>