fix warning in icc concerning non-ASCII character.

git-svn-id: http://root.cern.ch/svn/root/trunk@31874 27541ba8-7e3a-0410-8455-c3a389f83636

test/tstring.cxx

@@ -125,7 +125,7 @@ int main()
    Ok(21, !s.IsNull());
 
    // test IsAscii
-   s10 = "";
+   s10 = "\xb9";
 
    Ok(22, !s10.IsAscii());
    Ok(23, s.IsAscii());