Fix for Sourceforge bug #1450260 where this will assert on MSVC:

    boost::format("%s") % std::string()


[SVN r33443]

libs/format

@@ -1 +1 @@
-Subproject commit 5fd40ca2270fea108014c48cd5295584cab47b6b
+Subproject commit 4e94d4590890639763df2b817e387e9254e8aeb8