ocl: support empty "ptr only" UMat in Kernel::set()

add messages to avoid silent kernel destruction

modules/core/src/ocl.cpp

@@ -2998,7 +2998,11 @@ int Kernel::set(int i, const KernelArg& arg)
     if( !p || !p->handle )
         return -1;
     if (i < 0)
+    {
+        CV_LOG_ERROR(NULL, cv::format("OpenCL: Kernel(%s)::set(arg_index=%d): negative arg_index",
+                p->name.c_str(), (int)i));
         return i;
+    }
     if( i == 0 )
         p->cleanupUMats();
     cl_int status = 0;
@@ -3007,10 +3011,19 @@ int Kernel::set(int i, const KernelArg& arg)
         int accessFlags = ((arg.flags & KernelArg::READ_ONLY) ? ACCESS_READ : 0) +
                           ((arg.flags & KernelArg::WRITE_ONLY) ? ACCESS_WRITE : 0);
         bool ptronly = (arg.flags & KernelArg::PTR_ONLY) != 0;
+        if (ptronly && arg.m->empty())
+        {
+            cl_mem h_null = (cl_mem)NULL;
+            status = clSetKernelArg(p->handle, (cl_uint)i, sizeof(h_null), &h_null);
+            CV_OCL_DBG_CHECK_RESULT(status, cv::format("clSetKernelArg('%s', arg_index=%d, cl_mem=NULL)", p->name.c_str(), (int)i).c_str());
+            return i + 1;
+        }
         cl_mem h = (cl_mem)arg.m->handle(accessFlags);
 
         if (!h)
         {
+            CV_LOG_ERROR(NULL, cv::format("OpenCL: Kernel(%s)::set(arg_index=%d, flags=%d): can't create cl_mem handle for passed UMat buffer (addr=%p)",
+                    p->name.c_str(), (int)i, (int)arg.flags, arg.m));
             p->release();
             p = 0;
             return -1;