Fix warnings about uninitalized value

In some functors within Algorithm.h, the functor did not have a
default constructor that initialized its fields. This could in turn
lead to a warning about using it unitialized.

vtkm/cont/Algorithm.h

@@ -88,6 +88,11 @@ struct CopySubRangeFunctor
 {
   bool valid;
 
+  CopySubRangeFunctor()
+    : valid(false)
+  {
+  }
+
   template <typename Device, typename... Args>
   VTKM_CONT bool operator()(Device, Args&&... args)
   {
@@ -203,6 +208,11 @@ struct ScanExclusiveFunctor
 {
   T result;
 
+  ScanExclusiveFunctor()
+    : result(T())
+  {
+  }
+
   template <typename Device, typename... Args>
   VTKM_CONT bool operator()(Device, Args&&... args)
   {