AbstractPlainSocketImpl#close shouldn't set fd to null

pre-enso code expects fd to never be null. close() call should
only invalidate the fd object (sets to -1).
AbstractPlainSocketImpl#close is already invalidating the fd,
so I've updated the code to not alter the fd in close call

+ Updated every (fd != null) condition need to check if the
fd.valid()

Bug: 26470377
Change-Id: I69a2b9a9a5d7483952b3efe177bcd92e8f895bd1
2 files changed