Fix null argument handling during invoke from JDWP.

Fix a crash when passing null argument to a JDWP invoke.

Change-Id: I167f93f3a411f0de8458db3ba5bed3169d557ee9
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index 52a2141..2b82167 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -2712,7 +2712,7 @@
         if (argument == ObjectRegistry::kInvalidObject) {
           return JDWP::ERR_INVALID_OBJECT;
         }
-        if (!argument->InstanceOf(parameter_type)) {
+        if (argument != NULL && !argument->InstanceOf(parameter_type)) {
           return JDWP::ERR_ILLEGAL_ARGUMENT;
         }