In chain.hpp there is a variable named "null"

[herod.scott at gmail.com]

In boost/iostreams/chain.hpp on line 322 (of version 1.46 and others) there is a variable named "null". While that is legal C++, it is unfortunately all too common for people to place something like "#define null 0" somewhere else in their code base. It would be nice if that variable name could be made something like "null_device" which is less likely to cause a problem due to other code.