trace: don't leak the line prefix

The openssl app registers trace callbacks which automatically
set a line prefix in the OSSL_TRACE_CTRL_BEGIN callback.
This prefix needs to be cleared in the OSSL_TRACE_CTRL_END
callback, otherwise a memory leak is reported when openssl
is built with crypto-mdebug enabled.

This leak causes the tests to fail when tracing and memory
debugging are enabled.

The leak can be observed by any command that produces trace
output, e.g. by

  OPENSSL_TRACE=ANY util/shlib_wrap.sh  apps/openssl version
  ...
  [00:19:14]  4061 file=apps/bf_prefix.c, line=152, ...
  26 bytes leaked in 1 chunks

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8463)