Fixed one byte overread in njs_string_to_c_string().

Fixed one byte overread in njs_string_to_c_string().

Short strings are packed quite tight in njs_value_t, so there's
no one more byte to test.

    struct {
        njs_value_type_t              type:8;

        uint8_t                       size:4;
        uint8_t                       length:4;

        u_char                        start[14];
    } short_string;

With 14 bytes string this occupies 16 bytes, which is equal
to sizeof(njs_value_t).