OString::operator[]: make this consistent with OUString

Change-Id: If63362aba058bbcc0cc6bb1fae1c76005f1291d4

include/rtl/string.hxx

@@ -362,11 +362,7 @@ public:
       @since LibreOffice 3.5
     */
     sal_Char operator [](sal_Int32 index) const {
-        assert(index >= 0 && index <= getLength());
-            //TODO: should really check for < getLength(), but there is quite
-            // some clever code out there that violates this function's
-            // documented precondition and relies on s[s.getLength()] == 0 and
-            // that would need to be fixed first
+        assert(index >= 0 && index < getLength());
         return getStr()[index];
     }