Fix JDWP thread crash.

Fix a crash happenning when JDWP thread attempts to process incoming packets
from a closed socket. It happens when the runtime shuts down while JDWP thread
is still running and waiting for incoming JDWP packets.

When we close the JDWP connection from runtime destructor, we wake up the JDWP
thread. To avoid this, we call Dbg::Dispose before closing the connection.

Bug: 12578041
Change-Id: I8d03478b6c3f5f49e08899d476f292b7a401d5f3
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index 2141997..9194d73 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -464,6 +464,8 @@
 }
 
 void Dbg::StopJdwp() {
+  // Prevent the JDWP thread from processing JDWP incoming packets after we close the connection.
+  Disposed();
   delete gJdwpState;
   gJdwpState = NULL;
   delete gRegistry;