Fix another warning from MSVC

Author: d-frey

include/tao/json/events/ubjson/grammar.hpp

@@ -162,7 +162,7 @@ namespace tao
                }
 
                template< typename Input >
-               static std::size_t read_size( Input& in )
+               static std::uint64_t read_unsigned( Input& in )
                {
                   in.bump_in_this_line();  // Skip marker byte.
                   if( !in.size( 1 ) ) {
@@ -187,6 +187,16 @@ namespace tao
                   throw json_pegtl::parse_error( "unexpected ubjson high precision number size marker", in );
                }
 
+               template< typename Input >
+               static std::size_t read_size( Input& in )
+               {
+                  const auto s = read_unsigned( in );
+                  if( s > static_cast< std::uint64_t >( std::numeric_limits< std::size_t >::max() ) ) {
+                     throw json_pegtl::parse_error( "size too large for 32-bit platform", in );
+                  }
+                  return static_cast< std::size_t >( s );
+               }
+
                template< typename Input, typename Consumer >
                static bool match_high_precision( Input& in, Consumer& consumer )
                {