Fix VMStack::getClosestUserClassLoader to ignore NULL

NULL is the bootstrap loader, getClosestUserClassLoader was incorrectly
not ignoring it.

Change-Id: I8b18bb01d3a5ae8b9f34ccbbfa5fc25750d91716
diff --git a/src/dalvik_system_VMStack.cc b/src/dalvik_system_VMStack.cc
index 682aed9..fd2af39 100644
--- a/src/dalvik_system_VMStack.cc
+++ b/src/dalvik_system_VMStack.cc
@@ -60,7 +60,7 @@
   for (Frame frame = self->GetTopOfStack(); frame.HasNext(); frame.Next()) {
     Class* c = frame.GetMethod()->GetDeclaringClass();
     Object* cl = c->GetClassLoader();
-    if (cl != bootstrap && cl != system) {
+    if (cl != NULL && cl != bootstrap && cl != system) {
       return AddLocalReference<jobject>(env, cl);
     }
   }