Parsing of empty attributes

[zlomek]

While parsing an empty attribute, function parse_node_attributes() correctly creates a new attribute with value of length 0 but it sets the pointer into the buffer instead of to nullptr.
This does not harm in usual circumstances. But in case of cloning, it does. Then allocate_attribute() in clone_node() sets the value as value(val, internal::measure(val)) with length to the end of the buffer which is wrong.

Could you please check?

property_tree/include/boost/property_tree/detail/rapidxml.hpp

Line 2283 in 29a7f03

attribute->value(val, end - val);