dtrace: check if _handle property is set

Check that _handle is an object before trying to read its `fd`
property, avoids bogus values.

src/node_dtrace.cc

@@ -91,8 +91,12 @@ using namespace v8;
   } \
   node_dtrace_connection_t conn; \
   Local<Object> _##conn = Local<Object>::Cast(arg); \
-  Local<Object> _handle = (_##conn)->Get(String::New("_handle"))->ToObject(); \
-  SLURP_INT(_handle, fd, &conn.fd); \
+  Local<Value> _handle = (_##conn)->Get(String::New("_handle")); \
+  if (_handle->IsObject()) { \
+    SLURP_INT(_handle.As<Object>(), fd, &conn.fd); \
+  } else { \
+    conn.fd = -1; \
+  } \
   SLURP_STRING(_##conn, remoteAddress, &conn.remote); \
   SLURP_INT(_##conn, remotePort, &conn.port); \
   SLURP_INT(_##conn, bufferSize, &conn.buffered);