[boost][range] - Trac item 4226 - Implemented a safe_bool utility class that is intended to be refactored into a core area. Integrated this into iterator_range.

[SVN r67463]

libs/range

@@ -1 +1 @@
-Subproject commit efb7b50a8aa6a0e19a530ccec9a6f07bdc8ce2d7
+Subproject commit c506d2537fda683486c882262210e7c87deaf7a2