Fix error handling in pipelining test

Fix error handling in pipelining test

When an early error happens in the test_pipelining
function and idx == 5 the error handling would try
to call OPENSSL_free(msg), but msg is at that time
just a string constant in read-only memory, so a
crash would be the result. Fixed that by using
fragsize as an indication when to free msg.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21512)