Fixed 3 issues found by static analysis

modules/dnn/src/dnn.cpp

@@ -655,6 +655,8 @@ struct Net::Impl
         fusion = true;
         preferableBackend = DNN_BACKEND_DEFAULT;
         preferableTarget = DNN_TARGET_CPU;
+        blobManager.setPreferableBackend(DNN_BACKEND_DEFAULT);
+        blobManager.setPreferableTarget(DNN_TARGET_CPU);
     }
 
     Ptr<DataLayer> netInputLayer;

modules/dnn/src/layers/convolution_layer.cpp

@@ -783,6 +783,7 @@ public:
                 if( IS_POWER_LAYER(activ) )
                 {
                     Ptr<PowerLayer> activ_power = activ.dynamicCast<PowerLayer>();
+                    CV_Assert(activ_power);
                     a = activ_power->scale;
                     b = activ_power->shift;
                 }