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 #21512)

test/sslapitest.c

@@ -10851,7 +10851,7 @@ static int test_pipelining(int idx)
     ENGINE_finish(e);
     ENGINE_free(e);
     OPENSSL_free(buf);
-    if (idx == 5)
+    if (fragsize == SSL3_RT_MAX_PLAIN_LENGTH)
         OPENSSL_free(msg);
     return testresult;
 }