Fix read_cdr and take_cdr

[eboasson]

... and incidentally address the

/home/vsts/work/1/s/src/ddscxx/include/dds/sub/detail/SamplesHolder.hpp:89:9: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        ddsi_serdata_to_ser_ref(sd, 0, ddsi_serdata_size(sd), &blob_content);
        ^~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

flagged by the CI in #480 but not caused by it. (It was caused by eclipse-cyclonedds/cyclonedds#1947 adding the warn_unused_result attribute, and that's a good thing as it helped catch the bugs and the missing test.)