TCPStream issues

[adbridge]

1. When instantiating the class using the constructor:
   TCPStream(const socket_stack_t stack);

   there is no guarantee of a valid api being set. Thus if the connect method is then used, _socket.api
   could be NULL. Should check this within the connect method.
   If this is added then the method header should be updated accordingly.

2. Similarly for the setNagle() method. _socket.api could be invoked with a NULL value.
   Also setNagle calls set_option() which returns an error type which is then ignored. Shouldn't this
   error type be forwarded up to the caller of the method?

[ciarmcom]

ARM Internal Ref: IOTSFW-1616

[adbridge]

@bremoran bump.
Point 2 above still needs commenting / a decision made on