fix a bug in the accept call in library.js whereby if accept was called ...

fix a bug in the accept call in library.js whereby if accept was called with non-NULL addr and addrlen parameters a ReferenceError occurs because accept had a parameter of addrp but later used addr. Modified tests_sockets_echo_server.c to use non-NULL addr if TEST_ACCEPT_ADDR is defined and added test case to test_sockets.py