py: Fix mult by negative number of tuple, list, str, bytes.

Multiplication of a tuple, list, str or bytes now yields an empty
sequence (instead of crashing).  Addresses issue #799

Also added ability to mult bytes on LHS by integer.

py/objlist.c

@@ -131,6 +131,9 @@ STATIC mp_obj_t list_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
             if (!mp_obj_get_int_maybe(rhs, &n)) {
                 return MP_OBJ_NULL; // op not supported
             }
+            if (n < 0) {
+                n = 0;
+            }
             mp_obj_list_t *s = list_new(o->len * n);
             mp_seq_multiply(o->items, sizeof(*o->items), o->len, n, s->items);
             return s;