Use the context variant of getNullStream

Do not create a new queue to call finish in hipFree if none was
created earlier elsewhere.
Change-Id: I87bb191e6b186ddbe607ab29d11e3ae5bc2ac8e6

vdi/hip_context.cpp

@@ -103,6 +103,7 @@ amd::HostQueue* getNullStream(Device& dev) {
   }
   return stream->second;
 }
+
 amd::HostQueue* getNullStream(amd::Context& ctx) {
  for (auto& it : g_nullStreams) {
    if (it.first->asContext() == &ctx) {