[Ada] Fix memory leak in routine Wait_On_Socket

[Ada] Fix memory leak in routine Wait_On_Socket

gcc/ada/

	* libgnat/g-socket.adb (Wait_On_Socket): Fix memory leaks and
	file descriptor leaks.  A memory leak was created each time the
	routine was called without a selector (Selector = Null). Also,
	in case of exception in the routine a memory leak and descriptor
	leak was created as the created file selector was not closed.