Don't NPE when the remote peer address isn't yet known

This appears to be a race condition between the IO processor thread
and the application thread reading the connection table.  Apparently
the IO processors can register a session before they have finished
initializing it.

Bug: GERRIT-182
Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/src/main/java/com/google/gerrit/server/ssh/AdminShowConnections.java b/src/main/java/com/google/gerrit/server/ssh/AdminShowConnections.java
index aab8e73..6eb39a5 100644
--- a/src/main/java/com/google/gerrit/server/ssh/AdminShowConnections.java
+++ b/src/main/java/com/google/gerrit/server/ssh/AdminShowConnections.java
@@ -130,6 +130,9 @@
   }
 
   private String hostname(final SocketAddress remoteAddress) {
+    if (remoteAddress == null) {
+      return "?";
+    }
     String host = null;
     if (remoteAddress instanceof InetSocketAddress) {
       final InetSocketAddress sa = (InetSocketAddress) remoteAddress;