fix #4086

Signed-off-by: Nikolaj Bjorner <[email protected]>

Author: NikolajBjorner

src/ast/rewriter/seq_rewriter.cpp

@@ -1270,6 +1270,7 @@ br_status seq_rewriter::mk_seq_index(expr* a, expr* b, expr* c, expr_ref& result
             expr_ref a1(m());
             a1 = m_util.str.mk_concat(as.size() - i, as.c_ptr() + i, sort_a);
             result = m_util.str.mk_index(a1, b, m_autil.mk_int(r));
+            result = m().mk_ite(m_autil.mk_ge(result, zero()), m_autil.mk_add(m_autil.mk_int(i), result), minus_one());
             return BR_REWRITE2;
         }
     }