read_entropy_from_device on partial read will not fill buffer

read_entropy_from_device on partial read will not fill buffer

read_entropy_from_device() will loop in read until the desired number
of bytes are read from the device (/dev/random, /dev/urandom).  I have
observed that for /dev/random, if there is not enough bits available
for reading - it will return a partial read.  The code would loop in
this case, but never advance the location to place the new bytes -
hence the start of the buffer would be filled again - leaving the tail
end as stack garbage.

ticket: new
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19845 dc483132-0cff-0310-8789-dd5450dbe970