format zero length string msvc-8

[drjohnfoster]

Since Microsoft have deprecated most of the standard 
library, their debug libraries also check for unsafe 
pointers. The following code gives an assertion:

(boost::format("%s") % "").str();

In debug mode, basic_string<char>::append(0, 0) 
called from mk_str in feed_args.hpp is tested by the 
macro _DEBUG_POINTER, and decides the pointer is 
invalid even though the count is 0.

I imagine this toxic behaviour is repeated in a 
number of places. It would be nice if the boost 
library could work around this problem at a low level.