Merge pull request boostorg#59 from aiche/fix/parse_nan_deref_end-#6955

Check for first == last before dereferencing first

include/boost/spirit/home/qi/numeric/real_policies.hpp

@@ -116,7 +116,7 @@ namespace boost { namespace spirit { namespace qi
             // nan[(...)] ?
             if (detail::string_parse("nan", "NAN", first, last, unused))
             {
-                if (*first == '(')
+                if (first != last && *first == '(')
                 {
                     // skip trailing (...) part
                     Iterator i = first;