Fix incorrect length setting leads to out-of-bounds memory access

src/sql/engine/expr/ob_expr_substr.cpp

@@ -458,7 +458,7 @@ int ObExprSubstr::substr(common::ObString &varchar,
           res_len = min(length, mb_len - start);
           int64_t offset = ObCharset::charpos(cs_type, varchar.ptr(), varchar.length(), start);
           res_len = ObCharset::charpos(cs_type, varchar.ptr() + offset,
-              (offset == 0) ? varchar.length() : varchar.length() - offset + 1, res_len);
+              (offset == 0) ? varchar.length() : varchar.length() - offset, res_len);
           varchar.assign_ptr(varchar.ptr() + offset, static_cast<int32_t>(res_len));
         }
       }