tasn_dec: use do/while around statement macros

tasn_dec: use do/while around statement macros

Use the do {} while (0) construct around macros whose bodies are complete
statements (including one that has internal control flow!).  This is
safer and avoids any risk of misinterpretation if the macro is used in
an unexpected context.

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15143)