SWDEV-400448 - SWDEV-392872 - Add lock to NullStream

Fetching null stream's logic has changed earlier from amd::HostQueue
to hip::Stream. This seem to cause some timing difference between
checking for null stream and creating it due to which issues are
observed in multithreaded applications using default stream.

Change-Id: Ie02365dec537275d23a1d225de9811e2fd3a9c55

hipamd/src/hip_device.cpp

@@ -31,10 +31,12 @@ namespace hip {
 
 // ================================================================================================
 hip::Stream* Device::NullStream(bool wait) {
-  if (null_stream_ == nullptr) {
-    null_stream_ = new Stream(this, Stream::Priority::Normal, 0, true);
+  {
+     amd::ScopedLock lock(lock_);
+     if (null_stream_ == nullptr) {
+       null_stream_ = new Stream(this, Stream::Priority::Normal, 0, true);
+     }
   }
-
   if (null_stream_ == nullptr) {
     return nullptr;
   }