[io] ROOT::Detail::TKeyMapIterable: return ref in operator++()

Also make the iterator an input_iterator rather than forward_iterator,
since it does not satisfy all forward iterator requirements (in particular
"f i and j are both dereferenceable, then i == j if and only if *i and
*j are bound to the same object.")

Author: silverweed

io/io/inc/TFile.h

@@ -101,15 +101,15 @@ class TKeyMapIterable {
 
    public:
       using iterator = TIterator;
-      using iterator_category = std::forward_iterator_tag;
+      using iterator_category = std::input_iterator_tag;
       using difference_type = std::ptrdiff_t;
       using value_type = TKeyMapNode;
       using pointer = value_type *;
       using reference = value_type &;
 
       TIterator(TFile *file, std::uint64_t addr);
 
-      iterator operator++()
+      iterator &operator++()
       {
          Advance();
          return *this;