Check for null in LocalSocketImpl.SocketInputStream.available()

Calling available() on a local socket after close() will cause a native
crash. Other methods that pass LocalSocketImpl.fd to native code copy
the fd and check for null.

Change-Id: I2d0b573f34ae47a23009760bbb1c02933ab0a20e
Merge "Check for null in LocalSocketImpl.SocketInputStream.available()" into du44