fix getting a buffer in case a pool is not available

LibCarla/source/carla/streaming/detail/Stream.h

@@ -43,7 +43,10 @@ namespace detail {
     /// messages sent through the stream are big and have (approximately) the
     /// same size.
     Buffer MakeBuffer() {
+      if (_shared_state)
         return _shared_state->MakeBuffer();
+      else
+        return Buffer();
     }
 
     /// Flush @a buffers down the stream. No copies are made.

LibCarla/source/carla/streaming/detail/StreamStateBase.cpp

@@ -19,7 +19,10 @@ namespace detail {
   StreamStateBase::~StreamStateBase() = default;
 
   Buffer StreamStateBase::MakeBuffer() {
+    if (_buffer_pool)
       return _buffer_pool->Pop();
+    else
+      return Buffer();
   }
 
 } // namespace detail