Catch short documents in JSON input

When reading unnamed fields from JSON input, the member/value iterators
are incremented blind without checking if the end of the iterator has
been reached.

Record the size so that this can be checked against the current position
to ensure reading doesn't walk off the end of the iterator.

include/cereal/archives/json.hpp

@@ -483,16 +483,16 @@ namespace cereal
           Iterator() : itsIndex( 0 ), itsType(Null_) {}
 
           Iterator(MemberIterator begin, MemberIterator end) :
-            itsMemberItBegin(begin), itsMemberItEnd(end), itsIndex(0), itsType(Member)
+            itsMemberItBegin(begin), itsMemberItEnd(end), itsIndex(0), itsSize(std::distance(begin, end)), itsType(Member)
           {
-            if( std::distance( begin, end ) == 0 )
+            if( itsSize == 0 )
               itsType = Null_;
           }
 
           Iterator(ValueIterator begin, ValueIterator end) :
-            itsValueItBegin(begin), itsIndex(0), itsType(Value)
+            itsValueItBegin(begin), itsIndex(0), itsSize(std::distance(begin, end)), itsType(Value)
           {
-            if( std::distance( begin, end ) == 0 )
+            if( itsSize == 0 )
               itsType = Null_;
           }
 
@@ -506,6 +506,9 @@ namespace cereal
           //! Get the value of the current node
           GenericValue const & value()
           {
+            if( itsIndex >= itsSize )
+              throw cereal::Exception("No more objects in input");
+
             switch(itsType)
             {
               case Value : return itsValueItBegin[itsIndex];
@@ -546,7 +549,7 @@ namespace cereal
         private:
           MemberIterator itsMemberItBegin, itsMemberItEnd; //!< The member iterator (object)
           ValueIterator itsValueItBegin;                   //!< The value iterator (array)
-          size_t itsIndex;                                 //!< The current index of this iterator
+          size_t itsIndex, itsSize;                        //!< The current index of this iterator
           enum Type {Value, Member, Null_} itsType;        //!< Whether this holds values (array) or members (objects) or nothing
       };