Check for null socket name in socket_connect_local.

socket_bind_local checks if the name is null before using it, but
socket_connect_local doesn't, resulting in a segfault if you pass null
as the socket name. Check for this and convert it to
NullPointerException to make the crash more reasonable.

Test: am instrument -w -e class android.net.LocalSocketTest com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I67dedbe9ca1be6425e047496d0ead92eb7f64691