add an additional check for the capture operator

src/include/migraphx/op/capture.hpp

@@ -31,8 +31,16 @@ struct capture
     shape compute_shape(std::vector<shape> inputs) const { return inputs.front(); }
 
     argument compute(const shape&, std::vector<argument> args) const
+    {
+        if (f)
         {
             f(ins_index, args);
+        }
+        else
+        {
+            MIGRAPHX_THROW("CAPTURE: callback function is not callable!");
+        }
+        
         return args.front();
     }
 };