clover: Return 0 as storage size for local kernel args that are not set v2

The storage size for local kernel args can be queried before the
arguments are set by using the CL_KERNEL_LOCAL_MEM_SIZE param
of clGetKernelWorkGroupInfo().

The spec says that if local kernel arguments have not been specified,
then we should assume their size is 0.

v2:
  - Implement using c++11 member initialization.

Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>

Cc: 10.5 10.4 <mesa-stable@lists.freedesktop.org>

src/gallium/state_trackers/clover/core/kernel.hpp

@@ -175,7 +175,7 @@ namespace clover {
          virtual void unbind(exec_context &ctx);
 
       private:
-         size_t _storage;
+         size_t _storage = 0;
       };
 
       class constant_argument : public argument {