ALLJOYN-1617: Fix check for invalid endpoint in RemoteEndpoint.cc.
Remote endpoints can be invalid if they were created with the default
constructor or being torn down. Return ER_BUS_NO_ENDPOINT only if the
endpoint was created with the default constructor.

Change-Id: I6269bdb7f049ac0340d9a3b550b651097ab9313e
ALLJOYN-833 Updated the Alljoyn API change documents