Fix #1849 by calling device_global_buffer() unconditionally (#1862)

* Hold on to the CUDA buffer - second try
see: https://github.com/davisking/dlib/pull/1855#discussion_r314666348

* Fix #1849 by calling device_global_buffer() unconditionally

dlib/cuda/cuda_dlib.h

@@ -438,16 +438,12 @@ namespace dlib
                 const size_t bytes_per_plane = subnetwork_output.nr()*subnetwork_output.nc()*sizeof(uint16_t);
                 // Allocate a cuda buffer to store all the truth images and also one float
                 // for the scalar loss output.
-                if (!work)
-                {
-                    work = device_global_buffer();
-                }
-                cuda_data_void_ptr buf = work->get(subnetwork_output.num_samples()*bytes_per_plane + sizeof(float));
+                work = device_global_buffer();
+                buf = work->get(subnetwork_output.num_samples()*bytes_per_plane + sizeof(float));
 
                 cuda_data_void_ptr loss_buf = buf;
                 buf = buf+sizeof(float);
 
-
                 // copy the truth data into a cuda buffer.
                 for (long i = 0; i < subnetwork_output.num_samples(); ++i, ++truth)
                 {
@@ -471,6 +467,7 @@ namespace dlib
             );
             
             mutable std::shared_ptr<resizable_cuda_buffer> work;
+            mutable cuda_data_void_ptr buf;
         };
 
     // ------------------------------------------------------------------------------------