fix missing addref() in ocl::Context::create(str)

- fix https://github.com/opencv/opencv/issues/18906
- unable to add related test cases as there is
  no public access to Context:Impl refcounts

modules/core/src/ocl.cpp

@@ -2437,6 +2437,7 @@ public:
         if (impl)
         {
             CV_LOG_INFO(NULL, "OpenCL: reuse context@" << impl->contextId << " for configuration: " << configuration)
+            impl->addref();
             return impl;
         }