ocl: validate arguments in KernelArgs constructor

- don't use undefined flag=0. It should be CONSTANT instead.
- don't allow 'UMat* m=NULL' argument (except LOCAL/CONSTANT flags).
  This case is not handled well to provide NULL __global pointers.
  It is better to use '-D' macro defines instead (at least for performance)

modules/core/src/ocl.cpp

@@ -3155,6 +3155,7 @@ KernelArg::KernelArg()
 KernelArg::KernelArg(int _flags, UMat* _m, int _wscale, int _iwscale, const void* _obj, size_t _sz)
     : flags(_flags), m(_m), obj(_obj), sz(_sz), wscale(_wscale), iwscale(_iwscale)
 {
+    CV_Assert(_flags == LOCAL || _flags == CONSTANT || _m != NULL);
 }
 
 KernelArg KernelArg::Constant(const Mat& m)