Fix an access to grouping[grouping.size()].

AFAIK, this is valid for const strings but STLport (when _STLP_DEBUG
is defined) doesn't like it.
The problem reported by Bogdan Dragu.


[SVN r65279]

libs/conversion

@@ -1 +1 @@
-Subproject commit 00b609b498c12ebbfa4789ddcde7d741abee3d33
+Subproject commit c28efea0b3dba58b100e41bec54a4ca005a81306