Cygwin: fix memory corruption/SEGV if certain socket functions fail

Regression introduced with 2.11.0:

The failure paths in socket, socketpair and accept4 functions and
methods accidentally release *unused* cygheap_fdmanip objects.  The
subsequently called dtable::release method was designed to be called for
*used* cygheap_fdmanip objects only.  Using them on unused objects leads
to NULL pointer member dereferencing.

Worse, the inet/local accept4 methods only release the cygheap_fdmanip
object but neglect to delete the just created fhandler_socket_* object.

Fix this by removing the erroneous release calls in the aforementioned
failure paths and delete the fhandler_socket_* object in accept4 instead.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_socket_unix.cc

@@ -1639,7 +1639,6 @@ fhandler_socket_unix::accept4 (struct sockaddr *peer, int *len, int flags)
 create_shmem_failed:
 		  delete sock;
 		}
-	      fd.release ();
 	    }
 	}
       /* Ouch!  We can't handle the client if we couldn't